I know it means "You haven't seen anything yet", so can we also say "You ain't seen anything yet"? Which one is more common?
I think "You ain't seen nothing yet" is a lot more common than "anything yet" but neither is considered good English. It's colloquialisms, a typical Black American version of haven't/hasn't, a really warped form of English due to bad grammar. At times we use it as tongue in cheek when we really want to get someone's attention.