Mullen is a UF version of prime Mark Richt. Is that a bad thing? Well Richt is probably a HOF coach, but he didn't win any championships. If tomorrow both Diaz and Mullen were both free agents and I had to pick between them, would I pick Mullen over Diaz? Hard to say. I know that sounds bizarre but I'll tell you why it's not an easy decision. If by year 4, it becomes obvious that Diaz isn't the guy, he's going to be fired and Miami could go out and get a new HC who could potentially get Miami back onto a national championship track. (and since B. James is tied to Diaz, he probably goes too so we get a better AD). Of course, as a Miami fan I know Miami would probably blow the hire, but the hope is still there.
If Mullen was HC, he'd probably turn Miami into perennial 9-10 game winner, but we'd always be the bridesmaid and never the bride. The administration would never make a change if he's winning 10 every year, even though we know he'd never win a ring (just like Richt). So it would be another decade of not winning a national championship.
Also, I don't think much of SEC coaches coming to Miami. Richt showed you what happens when your program can't drop bags to get recruits. If UF hadn't dropped more than 250k large on Marshall (a figure high enough to make Bama balk), a 6-7 Miami team would have been taking UF's lunch money on the recruiting trail.
Miami needs an elite talent evaluator. That's not Mullen. You probably would have never heard of Kyle Trask if Felipe Franks doesn't get hurt.
It's not just UF hate. I've never been a big fan of Mullen.