There's nothing mental about lacking football skills.
The Cowboys drafted bad football players. Not good ones in need of a shrink.
Yes, there are probably players where it's more difficult to say if it's physical or mental. Why couldn't Alexander Wright catch a football in a game? He could do it in practice, but not in a game. I can't say for certain if that is a physical skill or mental/emotional deficiency.
I saw MartyB in person at training camp repeatedly put on a show in pass catching drills. He could catch better with 1 hand than a normal person can catch with two. He was big, strong and fast. Why did he fail as receiver in real games? Because unlike pass catching drills, he had to use his brain to know the proper alignment and routes. He didn't exert effort to learn because he thought that he was better than Witten and should be the starter.
I also saw Stephen McGee at training camp. He could really throw a football with both velocity and accuracy in practice drills; however, in games, he just couldn't pull the trigger. That is a mental issue. Not that he was stupid, he just couldn't handle it emotionally.
Bobbie Carpenter could do anything physically that Keith Brooking could do. Run fast, jump high, catch a football, flip his hips, etc..
Bruce Carter struggled in 2013. Does he lack physical ability?