I don't think the cowboys underachieved. I think they're right where they should be. Dak is massively overrated. Elliot is ok, but this is a passing league. They have no receivers and a has been TE. They're just not a good team and it shows. Nothing Garret could have done.
Well that may be what you think but that's not what just about everyone in Dallas and most outside of Dallas thought. Most thought this was the year they went deep into the playoffs if not the Superbowl. Those were the expectations nationally.
If they were overrated, it was on the defensive side of the ball. Dak and Elliot had very good years and their offense did well. The defense was exposed however. They VASTLY overperformed last year mostly due to a number of big defensive plays, caused a lot of turnovers etc. That setup up the offense and it worked. This year without the defense causing any turnovers the offense had to drive the length of the field far too often which they did for a lot of yards, but not for a lot of points. This is what happens when your defense is bad, it forces the offense to be great and they weren't able to do that.