I don't even know if you can consider him overrated as I don't feel he's ever made anything that I would consider moderately good.

He's known for re-releasing/re-cutting his movies mostly. His first film, the Dawn of the Dead remake was alright, 300 was ok, and Watchmen was a fairly faithful adaptation to the comic (other than the ending). but again he just recut most of that stuff post release to resell it.

He was handed the keys to the DC Universe and fumbled that massively. He tried to make Batman vs Superman a hybrid of The Dark Knight Returns and the Death of Superman and it just didn't work. it didn't make sense. And when I say a hybrid of both i'm being generous. It's a fight scene from DKR and a mish-mashed plot point from Death of Superman. It got so bad WB didn't even let him finish Justice League.