Here's the thing, I don't believe teams can effectively mask shitty defenses. Look at my Colts. Everyone that has watched at least one Colts game in the past 3 years knows that their defense is pretty bad, ESPECIALLY in stopping the run. They have to effectively win shoot-outs to win games which may get you into the playoffs(especially with how soft our schedule was a couple years ago) but will get you exposed by better offenses. So while Dallas' D might not have been great, it's been passable to keep them in games and make noise in the playoffs. You can also look at the Giants, Saints, Falcons, and to an extent, the Steelers to prove this point.
Blame WaterBomb XD