I would say he is inconsistent more then not being good. He looked great in the first half and very poor in the second. If he could put together complete games regularly then I'll be happy with him. Or he could end up as Cutler 2.0 where you see the possibility of greatness but he never gets there and he flat out costs you 1-3 games a year.
Not counting this last half he has looked light years better over the last four to five games then he did early in the season. His pre snap adjustments and being actually given the green light to audible are encouraging as well.
Edit:
The final score was also a lot closer then the actual gameplay indicated. Even with the offensive miscues from trubs and Cohen the bears should have won that by two scores if the refs hadn't gifted the Vikings eight points.
I also think some of the second half offensive woes the offense has had is on Nagy. First half against the packers, dolphins, and vikings we are moving the ball at will using the misdirects, bootlegs, and rpo's. Really up tempo and we are getting big leads. Then the second half comes around and the play calling becomes more vanilla. Lots of Jordan H up the gut and pure drop back passing. The coaching staff needs to learn to not let the foot off the gas pedal until the game is over.