Dallas does not have the best defense in the league. They're good, but Baltimore is ranked #1. Dallas' secondary has been picked apart a couple times. Their strength tends to be in holding teams down in points. They rank badly on turnovers.
I am probably the only person on this board who actually likes Dak Prescott as a QB, but as long as Garrett is his coach, he isn't going to light the league up any time soon.
The right coach could turn that defense into a powerhouse, along with re-tooling Prescott into being the better QB he showed promise in being with his rookie year. But as long as Garrett is there, that's not going to be happening.