Although Terry Bogard from Fatal Fury had made it into the game as a downloadable character, Masahiro Sakurai had initially planned to add Mai Shiranui alongside Terry as one of the cameos in the King of Fighters Stadium stage. In the Japanese Nintendo Direct presentation of Terry Bogard, he explained further that he worried that CERO (Japan's video game rating board) would raise the rating from A (for all Ages) to a higher one which was why he reconsidered having her in the game, joking that Super Smash Bros. was only for "good boys and girls".

According to Masahiro Sakurai, the reason Min Min was chosen as the playable Arms representative out of the game's cast is because Arms producer Kosuke Yabuki personally requested Min Min; Ninjara was among his top choices to appear in the game as well. Yabuki at one point considered asking them to add the entire cast of Arms' lead protagonists as alternate costumes or playstyles instead of just one character.

During the beginning part of Sephiroth's Supernova Final Smash where everyone turns to the Z-Axis to view Safer Sephiroth, the normally "flat" Mr. Game & Watch can be seen as a 3D model if he is caught in the attack.

By March 2019, with just 13.81 million units sold the that time, Super Smash Bros. Ultimate had usurped Super Smash Bros. Brawl (if one does not count the different versions of Street Fighter II and Super Smash Bros. for Nintendo 3DS and Wii U which Ultimate also surpassed) as the best-selling fighting game of all-time.

Sakurai and his team originally had different plans for a Xenoblade Chronicles 2 DLC character. He tried to include Rex as a playable character alongside Pyra, at first thinking of an Ice Climbers-like system where the player would control both characters at the same time, but this ended up presenting programming problems including one involving the game's memory capacity due to the size of the characters. He then had an idea of making Rex a background character whenever Pyra fights, in a similar manner to the Pokémon Trainer. He eventually felt this second method would also be taxing to the game and that fans would not be fond of it anyway so it was scrapped. He then came up with the idea to have Pyra be a transforming character (similar to how Zelda and Shiek were before Super Smash Bros. for Nintendo 3DS and Wii U) with her turning into Mythra and additionally have Rex appear only in taunts, Final Smashes, and other extraneous details involving the characters.

According to Sakurai in a Famitsu column, Sephiroth's inclusion as DLC in this game was his attempt to include more villains and/or bosses into the series as fighters, stating that the inclusion of such characters as playable was "rare". For one reason or another, he felt Final Fantasy VII's lead antagonist was an appropriate choice for this. He also stated that his game developing philosophy for these types of characters was that he wanted them to "feel like bosses" when playing as or against them.

When designing the DLC character Steve from Minecraft, most of the game's stages were redesigned to accommodate for his Mine neutral special move which allows him to farm certain materials on stage-specific surfaces to then use to craft stronger weapons. However, not every surface has been programmed to mine the materials expected to appear from the surface Steve is standing on. The most notable example of this occurs on most of the trees in the game, where they can produce Dirt materials instead of Wood.

As of March 2021, there are several mistakes, oversights, and omissions made by the "Move List" section of the pause menu. Some of them are as follows:

•Dr. Mario's Super Jump Punch (his Up Special) is described on his page as "hit[ting] repeatedly". however this is false as Dr. Mario's version hits only once at the start of the attack. Oddly, his base character, Mario, Has his version of the attack hit repeatedly but has a different description of the attack on his own page (the Tips menu corrects this).

•The text on Chrom and Roy's pages describing their counters says that both of the attacks send back the opponents attack "at greater force". However, while this is true in most situations, both counters have a cap of 60% damage that they cannot exceed or a minimum of 9.6% that they cannot go under.

•It says Isabelle's fishing rod grab can throw opponents up, down, or forward, with the menu making no mention of her being able to throw her foes backward with the same attack as well.

•King Dedede's Neutral Special description does not state that it can also automatically reflect projectiles, which is a new feature for the character in this game.

•Being a copy-paste of Simon's text, Richter's Holy Water descriptor states that it takes "fire" damage when his version of the attack actually has the "Aura" effect like Lucario's moves.

One of the two villains in World of Light, Galeem, may be based on the Seraphim, one of the rank angels in Judeo-Christian religions. Both Galeem and Seraphim have four pairs of wings and a halo, and both devour darkness with light, turning everyone (excluding the game's fighters) into spirits.

When Kazuya is in Rage Mode and the player is using a Pro Controller or a pair of Joy-Cons, the HD Rumble feature in those controllers will cause them to vibrate in a way that mimics the "thumping" of a heartbeat. This can also be felt with a GameCube controller connected to an adapter, although instead of "thumping", it creates a short, light vibration every couple of seconds due to GameCube controllers lacking the HD Rumble.

Minecraft World's soundtrack was taken directly from two other spin-offs within its series, Minecraft Earth and Minecraft Dungeons. The game's director Masahiro Sakurai stated in a Nintendo Direct that adding in the original music from the game would have been impossible due to the music's ambience not being suitable for the game, jokingly stating that it would "make players relax."

When the Hollow Bastion stage turns into the Dive to the Heart segment, the different Stations of Awakening in the background that reference key Kingdom Hearts characters appear to be censored of any reference to Disney characters such as Mickey Mouse, Donald Duck, and Goofy. However, these censored Stations are actually close replicas of the designs used on coasters that Square Enix produced in celebration of Kingdom Hearts' 15th anniversary, which also omitted the Disney characters.

When Kirby absorbs Sora and then uses his Magic, it is immediately clear that Kirby does not get a Keyblade of his own. Instead, Kirby uses an ethereal glowing weapon that resembles a keyblade but is entirely made out of light and sparkles that changes color depending on the spell. While this can be interpreted as being a restriction by Disney over the Keyblade's imagery, Masahiro Sakurai on Twitter chalked it up to being done for lore reasons as he said "...only Keyblade Wielders can use Keyblades..." implying that Kirby isn't "chosen" as barer of one and thus has to use the three spells differently.

A Famitsu column with game director Masahiro Sakurai revealed that Sora from the Kingdom Hearts series was able to get into the game because of a chance meeting that he had with a Disney executive at an unspecified awards ceremony where they expressed how cool it would be for him to be a part of the Super Smash Bros. roster. This soon led to negotiations between Nintendo, Square Enix, and Disney that eventually got the extremely highly requested character in Super Smash Bros. Ultimate. Before then, Sakurai admitted he and Nintendo thought getting the Disney-owned character was a total impossibility.

When Sora holds a battering Item or certain other items (such as a Mr. Saturn) he assumes the same idle stance that he did whenever he turned into Valor Form, one of the drive forms in Kingdom Hearts II that allows him to dual-wield two Keyblades, which in turn is also the basis for one of his alternate costumes in this game.

Xander Mobus who plays the Announcer and Master Hand and Crazy Hand in the English version of Super Smash Bros. Ultimate also reprises his role as Ren Amamiya/Joker from Persona 5 in this Smash game, making him the only English Language voice actor to play the announcer in a Super Smash Bros. game to also voice one of the game's playable fighters.

Ryo Horikawa (voice of Captain Falcon) revealed that when he was asked to record lines for the Min Min reveal trailer, he thought he was going to re-record alot of Captain Falcon’s lines in game. However, because Captain Falcon’s voice lines have been reused since Super Smash Bros. on the Nintendo 64, he was brought in to simply record ramen slurping sounds for the trailer.

Sakurai and his team were already working on the Byleth DLC before Fire Emblem: Three Houses was even released. The game director had to work with an early developer's build of the game, which he said took quite a long time to finish. This early build was in fact so Top Secret Sakurai couldn't even been seen playing it and he was even forbidden from taking it to his house.

In a Famitsu interview Sakurai revealed that he and his team wanted to introduce aerial Smash Attacks in this game in order to diversify the limited aerial combat of Smash Bros. However, they rejected this for various reasons, including the belief that such attacks were "too complicated" to implement and also that, in Sakurai's opinion, the system would also destroy the game and series' signature balance between casual and competitive gameplay.

The seemingly unique design for the Belmonts' Vampire Killer whip, with a metal chain, a morning star at the end, and the religious-looking crossguard, is actually very closely taken from two different sources: Castlevania: Lords of Shadow and the Pachinko game Castlevania: Erotic Violence.