I'd give him a solid B.
From a pure managerial standpoint, he's done a fantastic job. He manages his words well, knows how to say a lot without saying anything at all (it's annoying as a fan but great for a manager), and seems to genuinely care about players as people and try to accommodate them as best as he can. He's helped to make Dallas a place with a great reputation and somewhere that players want to play and stay in. This really is a key part of his job and something that can't be understated.
Drafting has been hit or miss, I'd say. It's gotten better recently, but early on, it was awful. We lost a couple of years of contending because of the 2013-2014 drafts. Look at how the youth now is supplementing the team so well, which is what is keeping us near the top of the standings. Imagine having that 3 years again when Benn and Seguin were near the top of their games. Heiskanen was a braindead choice to make, but the rest has seemed to at worst regress back to the mean.
Signings and trades have been above-average. He's had some misses but none have really crippled the team severely. Benn and Seguin's contracts might be bad but these were contracts that had to be signed. Any GM would've given those out and had they not, they would've been chased out of town. Everything else has been fine, mostly.
Cap management has been quite masterful. We've been near the ceiling for the last few years and yet we haven't had any real cap casualties yet, at least not of important core pieces. I don't think anyone is going to complain here.
Coaching has been bad, but this has been rehashed multiple times. He takes some of the blame, but not all of it. Hitchcock was owner interference, Monty's incident was largely out of his control, etc. I have been a fan of the assistants, and he at least is willing to make a change when one is needed.
I don't expect to see him be fired unless we have an embarrassing first round loss/miss the playoffs. I think making them and being competitive is enough to buy him another year or two. We're looking like a top 8 team in the league right now, we have elite goaltending that could get hot at the right time, and we certainly play the kind of grinding, defensive style that generally has success in the playoffs. Get there and anything can happen, especially as one of the better teams.
I've been hesitantly letting myself get more excited about our playoff chances. The fact that we have a high-quality 24+ man roster speaks dividends towards Nill's ability to supplement the (formerly) elite talent with great depth. It's unfortunate that things never lined up properly (and you can rightfully put that on Nill), but the pieces are there. I think we have at least a two-year window and possibly more depending on Robertson, Harley, Gurianov, etc. There's reason to be excited as a Stars' fan.
So sure, he isn't perfect, but it certainly could be much worse. I'm fine with things as is.