Actually, I'm not overly surprised by Dez's scant percentage of receptions.
If a WR can't get separation on such a regular basis, that's the end result.
Upon additional consideration and in fairness to Dez, I think it might help if Dak were to get accustomed to throwing to Dez more often with loft and to do so with accuracy. It seems to me that Dak is often prone to throwing the ball with velocity, making it more difficult to connect with Dez, who is frequently unable to attain separation. Dez appears to require loft more often to properly compensate for that disadvantage.
I believe if Dak got accustomed to lofting it more often to him and with accuracy, that it might very well aid their productivity. Maybe it's something they could work on to augment their effectiveness. It seems to me that that's where Romo had an advantage with Dez, in that he was somewhat more adept at throwing with touch, whereas Dak relies considerably more upon throwing with velocity. Anyhow, it's just food for thought.