I seriously thought this offense could be at least average.
That's all they had to do: be average.
The average number of points scored per NFL game is 22.8 points per team, per game.
That means, to be average, the Cowboys only needed 22 points. Surely, a top quarterback can muster up 21 points in a game even if you're lacking a WR. You still have a good back, your best buddy, a top 5 TE, a top 10 WR.
What else do you need to be avarage?
The defense did its job. The offense and particularly Dak failed the team.