To me he's alright but not great. Its hard to measure how great a coach truly is when they have a QB of Rodgers' caliber. A few playoff wins doesn't really do it. Greatness is doing what Bill Walsh or Belichick did with all time great QB's

. But there's definitely worse out there and he's got time to do a lot more.
You have to remember though, Rodgers was a talented yet flawed prospect in 2005. His mechanics were not what they are now and he was made of glass. Remember he was a Tedford QB abd they had to fix a lot of habits like holding the ball high when dropping back to throw.
MM put him through his QB school every offseason and since Favre didn't do offseason work, Rodgers was taking reps multiple years.
Tom Clements and McCarthy did a lot to get his development to where they were confident in him starting in 08.
Hell, he even got Matt Flynn to play semi decent in this system
And before he got to GB he was credited with helping other QBs in previous stops.