Bad Bananas are Bananas corrupted with the Megabug's power seen in the Donkey Kong Adventure mode of Mario + Rabbids Kingdom Battle. Originally created by the SupaMerge, they eventually corrupted Rabbid Kong when he ate one of them, giving him immense power and psychotropic behavior. He then ordered other Rabbids to stockpile this new power for him.
Bad Banana piles are stage obstacles in some of the game's levels; they do not affect the gameplay on their own, but all Bad Banana piles in a single level must be destroyed in order for Donkey Kong and co. to proceed.
After the group defeats Mega Rabbid Kong in battle, all of the Bad Bananas turn back into normal Bananas, causing Rabbid Kong to revert to normal.