Back in the day, total baldness was unusual — and lent itself to visually stereotyping a “villainous” predisposition. Other tried-and-true stereotypes: a conspicuous facial scar, some other physical deformity or albinism. (Has an albino ever played a good guy?) Now, of course, we’re more sensitive about “follicle shaming.” And most agree that a shaved pate is not a reliable indicator of morality.
As a matter of tradition, a modern Lex can still be bald. But a clichéd backstory (no hair = a path to evil) is probably not necessary.