He's a good, but not elite QB and needs to have help to succeed. They had that last year, but the OL took a step back, key skill players like Cooper and Crabtree struggling, switching the OC from Musgrave to Downing, and the defense's struggles he's got too many factors working against him. Not sure if anything has changed with him in terms of mechanics or decision making, though his struggles tonight were exacerbated by swirling winds, which will making both QBs look even worse tonight.
Interestingly, Carr's stats are very similar to Keenum's:
Carr 319/516 (61.5%), 3,383 YDS, 22 TD, 14 INT, 19 SACK, and 86.0 QBR*
Keenum 304/452 (67.3), 3,358 YDS, 21 TD, 7 INT, 20 SACK, and 98.1 QBR
The big differences Carr having lower accuracy and throwing twice the amount of picks, but otherwise they are surprisingly similar. Carr is doing this with an OL, skill players, and D that are all struggling. Keenum is putting his numbers up in a much more favorable situation. That's not meant as a knock on Keenum, but I was just surprised how similar their numbers were despite them dealing with very different situations.
*That is Carr's QBR through 14 games, it would now be lower after a bad performance against the Eagles. The rest of his stats are up to date.