It's a fun game, overall.
Sometimes things that are off the screen can still hurt you, notably frogs and bad mushrooms. I think the mushrooms in general should despawn after a few seconds, otherwise a bad mushroom can cut off a portion of the screen and be a nuisance for a fight. Actually, I think the mushrooms should take a couple of seconds to spawn, too, because I'll often be in the middle of a fight and kill an opponent, who almost immediately turns into a mushroom that I may not need (or that may be harmful).
Otherwise, this is quite fun!