Mario's a man of few words, so it's really impossible to tell whether he's actually intelligent enough to be a crazy chemist. He's definitely got the madness part down, though. His obsession with pills is worrying enough, but one day someone's going to discover his secret necromancy laboratory deep in the sewer of the Mushroom Kingdom. You think he found that dinosaur egg by chance? Not likely. Yoshi was created in a lab, mark my words.
On the other hand, one bad 'shroom trip is all it would take to send the good doctor into a mental battle royale with his alternate personality, Mr. Clyde. Sort of has a nice ring to it, doesn't it?