One hill I will die on is that it's always infinitely funnier for mad scientist characters to have an actual area of expertise, and the narrower it is, the funnier it gets. Like, they're mad scientists. They can branch out from it. But it's a very special thing to be able to point to the person who built a death ray/engineered life in their basement/etc and be able to confidently state what they got their PhD in.
"Doc Brown is a mad scientist" Well yeah, obviously.
"Doc Brown is a mad physicist" Now we're getting somewhere!
"Doc Brown is a mad nuclear physicist" Specifically conjures up images of a guy who spent his PhD doing a million repetitive cyclotron tests and eventually said fuck this, I'm building a time machine instead. Comedy gold.