He might be slightly overrated, but only slightly. He's been coach of the year twice, the first time being with the Orlando Magic and he got the coach of the year that year because he took an absolute crap team to a solid record. Go back and look at that roster he was dealing with the year he won it and you can see why.
His success with the Celtics, especially in the first year of the big 3 era, I think was a combination of a lot of factors...first, that team, especially looking at it in hindsight I think was PERFECTLY constructed, so Ainge gets all the credit there. But at the same time, we see all the time that just because the roster looks good on paper, you still need the right coach and system to make it all work so in that aspect, Doc gets a lot of the credit.
In terms of since being in LA, I think he's a good coach...but the problem is that he went to LA with the reputation of being almost a defensive genius, and the Clippers haven't exactly been worldbeaters when it comes defense, which I think shows that really the majority, if not all of the credit needs to go to Kevin Garnett for that. Can't underestimate the importance of Tom Thibodeau when he was an assistant under Doc as well.