A dingbat is an idiot: a person who's dumb, silly, or just empty-headed.
There are a lot of insulting words for someone who isn't very smart: moron, dummy, knucklehead, bonehead, etc. Another is dingbat. A dingbat isn't simply dumb, but also ditzy. On the sitcom “All in the Family,” Archie Bunker often called his wife Edith a dingbat. This is an insult, but it's humorous and not that offensive. Dingbats are also characters or ornaments used in typesetting that don't have any meaning. Those dingbats are also a little silly.