What I remember of the Stallone movie was that it was very.... Stallone-y. Sort of the concept the original Superman movie had where they wanted an unknown to play Superman rather than a star so that people would see Superman and not Actor-as-Superman.
Granted, Judge Dredd is nowhere near as iconic a character as Supes or as known but it can be hard to sell the idea of a character when people watching your movie are seeing an actor as himself. Stallone in Judge Dredd might as well have been playing his character from "Demolition Man" or "Cliffhanger" or "Over the Top" or something.
Carl Urban is pretty much an unknown so it was easy to "not see him" but an actor who's able to play a role other than "himself" would have been able to pull it off, mask or not.
The Stallone movie wasn't strictly "bad" from what little of it I remember it was just very... Stallone. And I suspect unsatisfying to fans of the character.