I think of All About Eve as more of a satire than a drama. Don't most people remember it for the one-liners? At least, it's not a serious film in the sense that Gone With The Wind is.
As for Sunset Boulevard, I'd categorise it as a film noir/black comedy. The humour is dark and grotesque for the most part, which is why people may not refer to it as a comedy, but it's there. That's what makes it so disturbing.
Comedy does not automatically equal lightweight, at least in my opinion.