And Luthor wasn’t even originally bald–he got mistaken for one of his henchmen early on. The bald henchman just stood out more, so it was easy to make him the leader. The fact that he was great contrast to Superman’s always perfectly coiffed (and oiled) hair was just a bonus.
Also, remember that Luthor’s thing isn’t just that he’s bald: he literally has no hair on his body at all. In many continuities, it’s implied (or even outright stated) that the reason he became evil is because so many people made fun of him as a kid. Perhaps that has something to do with the trope.
In the Classic Silver Age origin, young Lex is doing some experiment (nefarious, no doubt), his lab catches on fire, and Superboy uses his Super-Breath™ to blow it out. This causes chemicals to spill and causes Lex’s baldness. It’s an additional hatred-of-Superboy motivation.
*AND, on a related note:
*
Heard a podcast with Michael Rosenbaum (sp?), who said that he used to play nice guys, but as soon as he shaved his head to audition for Lex in Smallville he started getting Bad Guy Movie Offers.
(It may have been a factor that he did an excellent job of playing complex, nuanced, understandable evil. And John Glover did a great portrayal of his Evil Dad, with flowing locks…)
I think it has more to do with the fact that most men don’t wear hats any more. It used to be that the bad guys wore the black cowboy hats and the good guys (like Roy Rogers, Gene Autry) wore the white hats.
What if it’s Yul Brynner in a cowboy hat? He’s both bald and wearing a Black Hat.
He could be a Bad Guy (West World, Future World) or a Good Guy (The Magnificent Seven, Catlow)