Overrated is a subjective term and it depends who you talk to. It also depnds on your crteria for 'greatness.' In general I don't feel he gets overrated, although from a personal perspective I was a little miffed at his placing on our ESB survey we had a while back. He was a great fighter though, and an electrifying one. The fact that he is discussed so often many years after his death attests to this I think.
As I said before people often try to bash Foreman for fighting easy fighters early on in his career but never mention all of the barn storming fights Dempsey had. Fighters who had 0 wins coming into the fight with him. As somebody said he one the title from a god awful fighting in Jess Willard. Calling Dempsey a good fighter would be insulting. Even calling him a average fighter would be a insult. He was lucky to fight in the era he did because he wouldn't have even made a decent cruiserweight fighting todays fighters.
If anything he's underrated. When people who actually saw him fight were around, he was ranked right up there with Louis. He's top ten easily, personally I think he's higher.
Let's put it like this. The guys that rate him highly have him as a Paul Bunyan like mythical figure that cracked thunder and lightning with every punch. His actual record is somewhat underwhelming for his reputation and when you start sifting through the nitty gritty, its obvious that if he were around today he'd be flayed alive for his losses. I think the truth is in between. When he hit his prime patch he had many excellent wins, and the demolition job of Willard is shockingly brutal. I think Dempsey is one of those guys with the skills, power and the speed but who sometimes made sub-par choices in the ring - such as brawling with Firpo when I think he could have outboxed him with ease.
Generally, he is somewhere between overrated and very overrated. But that goes more to speak of those who idolize him and confuse events with fights. He was still, without any doubt, a great fighter.
He didn't fight the best fighters available during his own era , maybe his fault, maybe not, but it is fact. Watching him on film, I've seen nothing that any Heavyweight champs from the Louis's era through today would actually need to fear. So yes he is overrated in my opinion. Same with J.Johnson.
I think that the final verdict of history, has to be that he was over rated in his day, and in the eras following on from it. Today he is very under rated by many, who have a touch of "overreaction syndrome." The convert makes the worst zealot so to speak. His title reign does have some serious flaws, and most of his supporters admit it. However you slice it, his failure to meet Wills, is a serious problem. His pre title run was very impressive, and some of the bad boys of boxing, have built reputations on not much else. While his title reign was a disappointment, he did manage five title defenses, against legitimate challengers. When you put his pre title run, and his title reign together, not that many heavyweight champions can match his record. Head to head he was a monster. Perhaps the best heavyweight in history at overcoming a size disparity. As with guys like Liston and Tyson, you have to ask "what if he never lost focus?"