I don't think it is just player evaluation with respect to the draft that is an issue either.
I think Jerry tends to over value guys that the team drafted and sign them to contracts larger than their play warrants. You could question any of the recent contracts (Sensabaugh, Ratliff and Scandrick).
They obviously overvalued Roy Williams after botching that trade (initiated by his agent no less......
) and signed him to an unreasonable contract. Consider this: Roy Williams was in his fifth season with Detroit and had 1 1,000+ yard season when Dallas traded for him. He had never had a double digit TD season. Dallas rewarded him with a 5 year/45 million dollar contract with $20 million guaranteed. He would have been a free agent after the year and, if Dallas really wanted him, could have signed him then. No long term vision.
I think the whole owner/GM thing creates a culture in Dallas that isn't conducive to winning. Average performance will get you a contract extension. Players know they can step above the coach and go directly to the GM/owner.
I know some on here claim that a GM doesn't have anything to do with winning. I don't get that.....more talent makes winning easier. This team obviously hasn't been built to be good enough to win lately. However, I don't know how you can dismiss that the only times Dallas has improved under Jerry has been when he had coaches with a strong personnel background.