I'm guessing the reason some people hate him is because he was cocky and liked to showboat. Also, he was the media "golden boy" at the time, much the way Oscar has been in this generation. Some people don't like that.

As for the overrated thing, part of it probably stems from the fact that he got a couple controversial decisions. Many people thought Hagler beat him, and just about everyone thinks that Hearns beat him in their rematch.