Again, that isn't an uncommon thing, especially for a guy that is just trying to make the team with his snaps with the 3rd string players. Romo was horrible as a rookie in preseaon. Looked like a deer in the headlights and his feet were so nervous it looked like he was doing the Riverdance. Then there is the fact after injuries to Romo and Moore, Dak was with the first team as a rookie and was in heavy training to be the starter. It wasn't a normal rookie backup preseason.
In any case, you can make hard core definitive judgements after one preseason playing with the bottom players on the 90 man roster if you want, but I think that's a poor way to judge players