I don't think he's been a bad GM as far as player acquisitions go. As far as acquiring coaches, it's another story entirely.
The roster is pretty average. In terms of talent acquisition, I'd say Dallas has been relatively middle of the road.
In terms of managing the salary cap, however, Jerry Jones has been an absolute sideshow. He gets buddy-buddy with the players and then gives them MONSTER extensions with huge guaranteed money values years before their old contract expires, despite the fact that the player has no leverage and he's basically bidding against himself. Miles Austin and Jay Ratliff are cap killers on their own, but add them together, throw in all the other crappy contracts he's signed players to, and Dallas' salary cap is like a tiny car with its breaks cut, filled to the brim with clowns and hurtling headlong towards a cliff.
Seriously, Oakland thinks Dallas is doing a bad job of managing its salary cap.