It doesn't have to be mutually exclusive.

I don't think it is either of those. He is clearly not dumb nor do I think he is inherently evil. The thing is just that he became a billionaire at 23 and did not have to experience any of the grounding and setbacks that turn someone into a responsible adult.

He is basically a middle-aged college bro that always got what he wanted and never had to ask twice for something.

This is definitely a why not both sort of situation. In fact, I think being dimwitted is often associated with being evil, since goodness is (often) just rational self-interest.