Sorry, but "fun and comical" ≠ "dark comical."
Current Mundo is a murderous, masochistic, and ultimately unaware goofball. He's funny, in his uniquely twisted way, because he doesn't realize the damage he's causing as a "Doctor."
A "dark comical" Mundo, on the surface, seems to fly in the face of this. This type of humor requires the comedian to be aware of what they're doing or what is happening around him. (An example of this would be Saint Lawrence being executed by fire only to shout "Flip me over, I'm well done" after several minutes, thus making him a saint of cooks and comedians.) This would make Mundo less of a goofball and more of a psychopath, to be blunt.
We have enough "grim dark" champs. League doesn't need another one.