This comparison is irrelevant. We didn't get rid of Mullen. Mullen got rid of himself. He gave up and didn't want to coach anymore. If he had any morals he would have resigned and not dragged it out so long.
The only potential relative comparisons are to coaches we could have hired instead of Napier, e.g. Brian Kelly, Josh Heupel, etc.
Coaching hires are a crap shoot anyway... All the more reason not to do it very often.