One divisive element of Nickelodeon All-Star Brawl is how hard it leans into competitive play, which has been perceived as coming at the detriment of content aimed at casual players such as items and a story mode. According to Ludosity CEO Joel Nyström, Nickelodeon specifically wanted the game to be able to attract a competitive audience, and chose Ludosity as a lead development team due to the positive reception of their first platform fighter, Slap City, from competitive Super Smash Bros. players. 2 years later in 2023, the game's gameplay designer and community manager Thaddeus Crews would respond to a tweet from YouTuber TierZoo complaining about a focus on complicated competitive techniques in non-Smash platform fighters, as well as some replies to the tweet complaining about 1v1-based platform fighter design as a whole, with a clarification on why Ludosity's platform fighters have been marketed so directly at competitive players, as well as why he feels other games in the subgenre are marketed as such:

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.

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.

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.

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.

Competitive players of Nickelodeon All-Star Brawl nickname Rocko's grounded neutral strong attack, "Scream!", "Joe Biden" after a mondegreen interpreting Rocko's scream of "You're fired!" (a reference to the episode Power Trip, where he gets promoted to the boss of the comic book shop he works at and becomes mad with power) as "Joe Biden!"

NASB community manager Thaddeus Crews responded to a now-deleted tweet about the mondegreen with "canon", and later posted a stock render of Rocko with the caption "Joe Biden".

Alongside Garfield, Rocksteady from Teenage Mutant Ninja Turtles was originally going to be one of the two post-launch playable fighters in Nickelodeon All-Star Brawl - notably without his partner-in-crime Bebop, much like Michelangelo and Leonardo's inclusion without Raphael and Donatello. He was ultimately replaced by Shredder, and all that remained of his inclusion at launch was a placeholder stock image and the codename "hunter".

Rocksteady would eventually become playable in Nickelodeon All-Star Brawl 2.

Nickelodeon All-Star Brawl does not feature characters from live-action Nickelodeon series. According to Ludosity CEO Joel Nyström, this was due to the developers being unable to envision movesets for live-action characters (i.e. "I'd assume they don't feature a giant, two-ton hammer suddenly produced from your back pocket") and that many of the series did not air in Sweden. When asked about live-action content in an interview, he stated


Despite this, a developer from Ludosity has claimed that a live-action character was considered in the form of Alex Mack from The Secret World of Alex Mack, portrayed by Larisa Oleynik, but this was shot down by Nickelodeon.

Although not explicitly stated, it could also be assumed that live-action characters were off the table for likeness costs, given that All-Star Brawl was initially unable to afford character voices, and that actors involved in Nickelodeon sitcoms have been known to frequently get into extreme scandals that end their careers.

Nickelodeon All-Star Brawl does, however, feature a stage based on the game show Double Dare, and multiple aesthetic references to the iconic Nickelodeon slime which debuted in You Can't Do That on Television!