Are you saying that at no point in time was Bronco Mendenhall "cool"? This is the guy who surfs, rides a Harley, and named his kids Breaker, Raeder (pronounced "Raider"), and Cutter. Objectively, he was probably the "coolest" coaches in College football.
The reason some fans turned against him was because of QB injuries and multiple 9-4 type seasons.
Sitake is "cool" because he is new and different. For him to stay "cool" BYU is going to have to have a bunch of 10+ win seasons.
Such is the life of a head college football coach.