There was nothing lucky/flukey about Arizona's run to the SB - I think you totally underestimate how difficult it is to identify/develope the quality of QB that is necessary to contend for a SB. It is quite possible that Whisenhunt doesn't have that specific talent - that doesn't make him a bad HC.
Belichick failed miserably in Cleveland because he couldn't identify/develope a QB there. He wasn't doing much better in New England with Bledsoe. Brady was inserted because Bledsoe was injured - not because Bledsoe was playing bad. A slight fluke in history is the difference between Belichick being hailed as a genius and another failed tenure as an NFL head coach.
I'm not on any Whisenhunt band wagon - I'm simply stating that an opinion that Whisenhunt is a bad HC because of the end result in Arizona is an uninformed opinion.