Flanderization

Battle of the Multiverse is a franchise that is two things: crossover and gore. Many of the characters have their personalities exaggerated to cause more deaths. While some are to expand some bland character's personalities, others are to make exaggerated humor, make stupid and absurd conflict, or cause more gore.

SpongeBob
SpongeBob was basically an underwater, sea-sponge everyday Nice Guy. The "earnest everyman" aspect of SpongeBob's personality has been completely downplayed to make him gratuitously obnoxious and annoying rather than merely naive and eccentric. Now he's gone into a psychopathic who cares less about his friends deaths and injuries, rivaling Handy in apathetic nature.
 * It's shown the most during his driving. Occasionally, SpongeBob is shown to drive a car perfectly, but when a deaths must happen, he's a reckless driver who kills other characters.
 * SpongeBob's kill count is very high because of his reckless personality. It gets to the point where he rivals Fliqpy in terms of kills.

Cuddles
Cuddles is reckless and a little mischievous but he's still friendly, naive, and cheerful. In BOTM however, he is made more reckless just to provide more on-screen deaths or more conflicts. He also makes SpongeBob's life somewhat of a living hell because he always have to make sure that he stays out of trouble. This also adds a mischievous attitude towards his personality. He somewhat maliciously pull pranks on others just to get a laugh. In HTF, he does pull some pranks and he is responsible for Toothy's and Flaky's deaths on occasion, those pranks were harmless and it did not mean to kill his two best friends. In BOTM however, he murders or injuries his friends intentionally or as a sick joke.
 * With that being said, he's also painfully naive. He falls for other character's schemes and always makes SpongeBob and his friend a living hell in this regard as well.

Kirby
Kirby's violent side is made more apparent. In a lot of episodes, he resorts to violence, which is especially seen in the spin-off, Chubby Short Hungry Aliens. He is the least flanderized f the three main mascots, but it still leads to some severe results.
 * What is flanderized however, is his appetite. He sometimes endangers his friends so he can eat his snack. Some games in the canon series does portray Kirby's appetite in a negative matter but that's the extend of it. Kirby occasionally even eats innocent bystanders.