I don't think Carbs is stupid or a bad coach. He might be a little conservative, but he also is hamstrung with limited talent resources.
He is the only coach in history to win the major coaching award at all three levels. (NHL-Jack Adams, AHL-Loius Pieri, ECHL-John Brophy) Those didn't come by accident or him getting lucky with talented teams. (Okay last year everyone overperformed so he there some luck to it)
He is no dummy, and maybe because of his time in ECHL, AHL, he maybe limits the young guys more, it's hard to say.
I think one of the reasons Caps moved to him instead of an established guy is that they knew they were headed for rebuild (of sorts) and Carbs is a younger guy who has helped develop guys at all levels. They got lucky last year and had a better season than expected so it altered the narrative a bit last year.
Ultimately everything is the coaches responsibility but its tough when you have 2-3 guys playing well, and a bunch of guys underperforming, and a bunch of guys that just are not that great.