There are lots of coaches that have a poor reputation. Sometimes it is deserved but sometimes it is not. Take Fred Hoiberg. People thought he was trash. Well guess what Jim Boylen is a lot worse. Take Mike Brown. People think he is a C or D grade coach. But when Kerr went down, he showed he is a decent coach. I could go on with names like Kevin McHale, Scott Brooks, Kenny Atkinson, Tyronn Lue, Mike Malone, Jason Kidd, etc. So who would people consider the coach with the biggest ability to reputation differential?
Spoelstra. You look at that roster and you have to scratch your head at how they're even a .500 ball club much less leading their division right now. Okay, their division sucks but he's been working with mismatched parts for the past few years now and still manage to have a respectable team out there.
Spoelstra is a respected but not elite level coach. He probably has B+ ability and B reputation so the gap isn't that much. This is similar to Scott Brooks. The coach with the biggest gap might be Tyronn Lue. He is considered a D level coach but has at least B- ability somewhat like Hoiberg.
You asked about being underrated, not who's an elite coach. No one mentions Spoelstra as a great coach because people thought he rode LeBron's coattails and yet his team is always right there in the playoff mix. Tyronn Lue and Mike Brown without LeBron = unemployment. I will be very shocked if Lue gets another NBA head coaching job again. Same for Hoiberg
Dwayne Casey who got fired after Toronto went deep in the playoffs! Tyron lue who went to the finals with lebrin and the cavs lebron said his play calling was bad. Tom thibadeaux who got fired after the Jimmy Butler trade with timberwolves.
I'm watching Dallas games this year because of Doncic and I must say Carlisle seem quite overrated to me. As someone who hasn't watched much Dallas before this year, I may not be the one to criticize him. But how else can you explain his starting five has worst net rating of all starting fives that logged at least 250 minutes this season? Maybe Wesley Matthews has to be showcased. That is the only logical explanation.