Ren & Stimpy were made a duo fighter in Nickelodeon All-Star Brawl to allow them to interact with one another, something that wouldn't be possible if they were two separate characters. Ludosity CEO Joel Nyström noted that it also allowed development time to be assigned to one extra fighter.

In December 2023, information spread about Warner Bros. Discovery, which owns Cartoon Network among other major animation brands, potentially merging with Paramount Global, which owns CN's long-standing TV animation rival, Nickelodeon among other animation IPs. Although this deal would not come to fruition at the time, Nickelodeon All-Star Brawl game designer Thaddeus Crews commented that he would take the chance to work on a Cartoon Network vs. Nickelodeon fighting game "in a goddamn heartbeat".

Originally, Reptar was going to spit cereal from his mouth, but Nickelodeon insisted that he shoot fire instead.

Out-of-bounds on Omashu are two floating textureless rectangles, while Space Madness has an out-of-bounds cube. These were both possibly used during development.

Hugh has an unused model of a spear, referencing the popular caribou hunting chant scene from The Adventures of Jimmy Neutron: Boy Genius episode "Jimmy on Ice". While it is fully textured and loaded into the game whenever Hugh is, it is not used.

Various models and textures of Ittle Dew from Slap City are inside the files of Nickelodeon All-Star Brawl, as she was used as a placeholder during development.

In launch versions of Nickelodeon All-Star Brawl, Garfield's stage Sweet Dreams was discovered before his official announcement in an unfinished form which takes place at day time, unlike the final version which takes place at night. Additionally, a thumbnail was discovered for an extremely early version of the stage that takes place at sunset and has an opaque lasagna tray. When Sweet Dreams returned to Nickelodeon All-Star Brawl 2, under the name Food Dream, the daytime setting for the stage would be restored.

Nickelodeon All-Star Brawl 2 has been noted as having almost zero similarities to the game it is a sequel to from a gameplay perspective. Gameplay and moveset designer and community manager Thaddeus Crews would address this disparity in a short Twitter thread shortly after the sequel's launch by reassuring fans who prefer the first NASB that the sequel was not intended as a replacement for the first game:

In the final update of Nickelodeon All-Star Brawl, Patrick's command grabs gained a glitch that made them unable to penetrate through shields, negating their primary purpose. When asked about fixing Patrick's grabs after the launch of the game's sequel, gameplay and character designer Thaddeus Crews responded "I would if I could".

In 2023, a set of extremely early Nickelodeon All-Star Brawl screenshots were shared through Discord by a developer, showing an aesthetic closer to Slap City and early versions of SpongeBob, Oblina, Leonardo, The Dump, Jellyfish Fields, and - most notably - Rooftop Rumble, based on Shredder's lair from the 2012 Teenage Mutant Ninja Turtles show instead of the 80s TMNT (despite the 80s turtles having already been decided on this point). Also among the images was Hardcore Chores, Rocko's DLC stage, suggesting that a Rocko's Modern Life character may have been intended for base game.

A Twitter post made on the Nickelodeon All-Star Brawl Twitter upon the release of the Garfield DLC shows that his bib alternate costume was originally supposed to be plain white with stains of lasagna tomato sauce instead of clean with red checkers.

Lincoln Loud's neutral water gun move "The Splasher" had to be reworked during development; the original version of the move stunned opponents and could be cancelled into most actions in the game. This was a flawed move design however, as players could simply cancel the move into a waveland and then perform it again, making an infinite combo. If this move stayed in, Lincoln would have had to be banned from early competitive play, much as Michelangelo would end up being. In the final game, the water gun pushes opponents and cannot be cancelled.

If the player holds the button after performing Hugh Neutron's grounded up light and then presses left or right on the stick, they can skid Hugh very slightly. Hugh moves slightly further with every skid, and, as there is no cap to how far the skids can go, he can become so fast that he teleports from one side of the stage to the other with a single skid. This has no practical use in a battle due to the amount of skids required to begin teleporting, as well as the skid count restarting every time Hugh uses the move, but appears to be a deliberate feature implemented by the developers, as every time Hugh skids, he makes a short squeaking noise.

During development of Nickelodeon All-Star Brawl, Patrick's gravity accidentally ended up being set to 420 in certain unspecified circumstances. Thaddeus Crews stated that it was kept that way as a joke.

Nickelodeon All-Star Brawl's movesets are modified through mathematical functions. Due to the monotony of assigning these values, certain modifiers' values are set to be pi solely as a joke. Thaddeus Crews, who added the instances of pi, stated in a Reddit post about the use of the number:

When used on the Wild Waterfall stage, the Suck-O-Matic can suck up the log platforms, altering the layout of the stage.

If you pause the game on Jellyfish Fields, Gary's eyes will start to bizarrely drift away from one another. When you unpause, they snap back in place.

There exists an unused model in Nickelodeon All-Star Brawl of Patrick's head after being shoved through a trombone, in reference to the SpongeBob episode Band Geeks.

One of Ren's voice lines on Nickelodeon All-Star Brawl, used for his and Stimpy's grounded down strong attack where he slams down Stimpy as if he were a hammer, is a reference to the streamer CONEY, who in his post-launch NASB gameplay referred to the move as "the STEEEEEEEEEEEMPY", with Ren's second original series voice actor Billy West replicating the cadence of CONEY's scream.

While most other characters in Nickelodeon All-Star Brawl 2 recieved revamped movesets, Jenny's moveset is nigh identical to her moveset from the first NASB aside from inputs that didn't exist in the NASB 1 moveset template. This is due to moveset and gameplay designer Thaddeus Crews considering Jenny's moveset to be one of his favorite kits from the first game.