The problem with Garrett is that, to me, I would like to see something in games where he seems to actually make a decision, other that putting up one finger for the extra point after touchdowns. I am open to the possibility he does all of his work during the week in meetings, etc. and lets his assistant coaches run the nuts and bolts of the game, which is the way most coaches do it. Landry called offense and defense in games, but that is conspicuous by its presence.
For example in one playoff game during Jimmy's tenure the other team was driving the ball down the field, and Jimmy went to Wannstadt (if memory serves) and yelled to him "I want a blitz, and I want it NOW" A blitz was called and the drive was stopped. All we see Garrett do is stand and watch the plays, maybe congratulate a player on a good play.
There are all kind of coaching styles, Madden was a real ball of fire on the sidelines, but got an ulcer and had to quit coaching, as some reported. Landry showed little emotion most of the time, that was his personality/style, and it worked. Belichick is about the same, and he's won 5 SBs. So not being emotional on the sidelines can work, doesn't mean he's not involved.
Since I don't know how much Garrett is involved with on-the-field decisions, I have to judge him based on results. 1-2 playoff record, and those 3 games in 10 years. Not running the ball when it was clearly the best play. Poor clock management. Rushing 3 players against Rodgers with the game on the line.
Sure he's had some good seasons, and if he had a lot to do with Dak's great first year, that's a good sign.
But I don't see he's proven to be a good coach, if by good someone means he doesn't go 0-16 or 4-12 year after year, then yes he is. I place him as a mediocre head coach at this time, if he's here a few more years and the team improves to being a SB contender several years in a row or wins a number of playoff games, then I'll gladly say I was wrong...