Holmgren was transformational in a way that MM never had to be and probably couldn't be. He was responsible for a full blown culture change. He had to deal with a veteran roster and pulled off perhaps the greatest reclamation project in NFL history. Beyond that, he's proven his coaching prowess with other teams.
He was perfect for his time and place, but MM has been as well. McCarthy came to the Packers in an era of offensive explosion. Rule changes benefit all teams the same, but MM has coached, schemed, play-called and/or otherwise coordinated 2 of the top five most efficient/potent offenses in the history of the league (2014 and 2011). Holmgren rightly receives credit for corralling a young, wild Brett Favre and helping him reach his enormous potential, but McCarthy was tasked with corralling an older superstar Brett Favre who seemed to have become used to not being coached and held accountable. And THEN he had to deal with the transition from his HOF QB to another talented QB, all the while maintaining control of the team and not allowing the pressure to derail Rodgers' career before it really got going.
Holmgren came in with a ton of fanfare and high expectations and because he delivered on those expectations, it's easy for fans to sing his praises. Sometimes I think McCarthy came in with such low expectations that people are reluctant to see just how much he's shattered them.