I'm focused more on the last 4 seasons than his early career, so if you look at the results from the last 4 seasons he and the coaching staff have been able to coach up the talent they've recruited.
2017:
Final CFP Ranking - 11th
Avg. Recruiting ranking for the previous four seasons (using Rivals): - 12th (2014-2017)
2018:
Final CFP Ranking - 5th
Avg recruiting ranking -12th
2019:
Final CFP Ranking - 12th
Avg. recruiting ranking - 13th
2020:
Final CFP Ranking - 4th
Avg. recruiting ranking - 15th
So basically if you take team recruiting rankings as an indicator of future team performance, Kelly's teams have performed to expected levels for 2 seasons (2017 and 2019), and way above expectations for 2 seasons (2018 and 2020). I know recruiting rankings aren't absolutely accurate but they do give a general indication that Kelly has been able to get more out of the talent he's recruited than could have been expected. Having said all that, it does not excuse the inability to elevate the level of talent being recruited. It's inexcusable. If he had been able to elevate his recruiting to top 5 classes, there's no telling how close he could have come to challenging Alabama, Clemson and Ohio State.
So overall, I'd give him a Very Good. The inability to recruit better kept him from being Excellent.