Statistically Kirk was fairly close between Zimmer and KOC, the main difference was the wins in KOC's era due to Kirk playing really well in the 4th quarter, though notably was preceded by playing poorly in the 2nd-3rd quarters. 2019-2020 under Stefanski and G Kubiak were his best statistically.
IMO JJM is way more talented than the rest of that group. On another forum, somebody asked this:
To which I responded: In college, it was his accuracy in the intermediate range, consistently converting 3rd downs (best 3rd and long conversion rate among college QBs that year), playing fairly well when pressured, played in a pro offense under center, and had the mobility to run for some first downs. It's kinda incredible how none of that has translated to the NFL. Not to mention he had zero red flags when it came to durability and now he's incredibly fragile.
The whole "JJM was hidden on a run-first super offense" isn't exactly true, heck, comparing him to Fernando Mendoza, JJM only threw one less pass per game comparing their final years and Michigan had zero NFL caliber WRs or OL to support the passing game. They blew out most their opponents in their championship year, so it was natural for them to run the ball a ton. Michigan's QB play before and after him were far worse on a supposedly easy-to-play offense.
I can be argued down to JJM being a high 2nd round caliber prospect without the use of hindsight. Plenty of prospects with relatively clean prospect profiles bust, it doesn't make the scouting reports wrong in hindsight. You can't project all of a prospect's strengths failing to translate and his maturity and durability to be far less than expected, that's just a risk that comes with the pick.
This regime has had a knack with everything going wrong for a prospect (see Cine, Booth Jr, Asamoah, etc). You just think having the likes of Josh McCown and Kevin O'Connell would get some level improvement out of JJM, even a little bit? Heck, even Brosmer looks worse after dominating last preseason. KOC can get a solid baseline level from veteran QBs, but you know who else can do that? Most other head coaches, including Mike Zimmer.