From 2010 through 2016, I would have said mediocre to good. While he had a great season in 2012, an excellent season in 2015, and a very good one in 2013 (I give him that grade because he squeezed the absolute max performance out of Tommy Rees), three of the others (2010, 2011, and 2014) should have been at least BCS-bowl seasons if it weren't for some really bad coaching decisions.
2010: Trying to turn Dayne Crist into a dual threat quarterback...
2011: South Florida and Michigan were easily winnable, but scaling back the offense all but killed us.
2014: Brian Van Gorder...
2016: Giving Malik Zaire playing time, and costing us the Texas game, not to mention, Zaire's choking and foolishly throwing a short pass against USC to end the game, not even trying to make a Hail Mary pass.
For that matter, Brian Van Gorder's idiocy cost us a playoff berth in 2015. Who else would have been dumb enough to consistently play a Cover Two / Cover Three scheme against a run-heavy Stanford offense? If it weren't for Jaylon Smith's unearthly talent, that season would have been significantly worse.
From 2017 through 2020, I would call him an excellent coach. He finally got rid of a lot of dead weight in the way of Van Gorder and Longo, and started playing to the strengths of the team, rather than trying to force the players to fit his rigid mold. Two playoff appearances have solidified Brian Kelly's position among the top tier of college football coaches.
Unfortunately, that's still a notch below the best of the best.