Serious villains are easier to use for "plug and play" stories where the main hero doesn't have to dominate the story since the villain can define a great conflict, while campy villains tend to encourage more personality from the heroes because the conflict usually isn't that strong. And of course, in comics there's actually plenty who kind are "both"; Dini's version of Joker is firmly more funny than most other variations on him in modern comics while still being terrifying, and the Flash's Rogues Gallery is kind of a glorious combo of campy and serious.