Awful writing in addition to Hayden Christiansen really brought the second and third movies down. Those movies are a lot more decent if there's at least tolerable dialogue and a less shitty Anakin Skywalker.
He was far worse in Ep 2. The guy had zero to work with, but he's only an average actor to begin with. I don't think anyone could have worked with the dialogue (look at Portman), but a better actor would have been passable.
As UKK a said, though, Clone Wars saves Ep 3 by a lot.