I remember when it first came out, I was watching it and thinking, "wow, you are in CA, you are looking for a young male actor, blonde, to play this pivotal role and this is the best you could come up with?
That whiny douchebag is supposed to be Darth Vader? Sure! Portman is almost as bad.
I actually thought the child Anakin was not too bad - definitely not in the top 10 of bad Star Wars performances.
He was a bit obnoxious/precocious, but I think the role called for it.
The kid was in other films and did fine. Sometimes it ain't the acting, but the screenplay and directing that destroys a performance.
Specifically, in the case of the Star Wars movies -- the only one that's really a good movie in its own right is Episode V: The Empire Strikes Back. Why? Because it's the only one that Lucas neither directed nor wrote the screenplay for.
With the exception of Harrison Ford, the original Star Wars (Episode IV) was horribly acted, very wooden. Lucas directed and was the sole screenplay writer.
Return of The Jedi wasn't horrible (replace the Ewoks with Wookies like they were originally supposed to be...) -- but Lucas was only co-wrtier of the screenplay there, and didn't direct either.
The "prequel" trilogy has Lucas' prints all over them (co-screenplay writer for Episode II, but otherwise all Lucas in writing and directing) and they can easily be characterized as cartoonish stories with bad acting.