I'd argue he's part of the reason they are not built to score a lot of points. Not the only one, but definitely part of it.
Both games in which the defense didn't play great, they lost.
He hasn't been terrible, but has not been consistently good by any stretch. Sure, he had his moments, but that was basically his MO with the Bears in that he would flash enough to get your hopes up, but never progress his game to the next level. Just ask Bears fans.
He's never going to be a QB a team has a top 10 scoring offense with.