You mean the 2012 Eagles team that finished 4-12 with the 29th ranked defense and 29th ranked offense was one of the best teams in football?
Kelly has ruined the team with his personnel decisions, no doubt. As for his coaching ability? Yeah I think he's pretty good there.
Eh...one of the best might be an overstatement. But it was a good team missing a QB when Reid left. You have to keep in mind the injuries and poor QB play they had in 2012. They still had DeSean Jackson, Jeremy Maclin, Brent Celek, LeSean McCoy, Jason Peters, Evan Mathis, Todd Herremans, and Jason Kelce on that team.(and more) These guys were all key contributors for Kelly on offense. In addition to that, Reid drafted Fletcher Cox, Vinny Curry, Brandon Boykin, and Mychal Kendricks in that draft. He also had brought in Cedric Thornton, DeMeco Ryans, Trent Cole, and Nate Allen in prior years. They had a ton of youth on defense that ended up developing into key contributors for Kelly.
If you look at the 2012 Eagles, they had a ton of talent on offense (except for the QB position) but suffered a ton of injuries. And on defense, they mainly had young players or overpaid free agents.
I'm not saying the team was perfect...but it was a very talented team. Reid made some errors going all in during FA. But the 2012 Eagles were not nearly as far off as their record appeared. They were a QB, some time/experience on the defensive side of the ball, and not getting decimated by injuries offensively from being a good team. Kelly didn't go in there and remake the team. The offense was almost unchanged.(Lane Johnson as the starting RT and Zach Ertz as the #2 TE were the only major changes) And yes, it's impressive what he did with Nick Foles that year...but that obviously didn't continue after defenses adjusted to his scheme. But they already had one of the most talented offenses in the league. The defense was a pretty mediocre unit but they did have some young talent.
Basically, it was a quality team...and he tore it apart. But I'll agree that one of the best was an overstatement. With a good QB, they would have been. But they didn't have a good QB.