Three of those 6 bowl wins came in the last 5 years. One of those top 25 finishes came in the last 5 years. Bronco's 0.631 record in the last 5 years is still, far and away, the second best record under any coach (not including Bronco's first 5 years). And that's all with a notable increase in strength of schedule.
I don't think anyone on here would disagree that had Hill, Williams, and half the defense not gone down to injury last year (worst year I can ever remember in that regard), we would have finished in the top 25 yet again, and probably had another bowl win. In fact, the last 5 years have been plagued by QB injuries (more than probably any other 5 year stretch in history). At BYU, where the QB position is so important, that's very significant. And we still managed to have no losing seasons and go to a bowl every year and win most of them. I wouldn't call that mediocre.