He certainly did waste opportunities. And your comparison to Wesley Clark is a good one. McClellan was a great organizer, but no fighter (the Peninsula Campaign for example). And I believe the people recognized that during the 1864 Presidential election. Not to say that was the only reason Lincoln was re-elected.
I don’t know if it’s true or not but I’d read that Lincoln was re-elected largely because of the military vote. Now this always sort of confused me because I’d read that “Little Mac” was extremely popular with his troops. On the other hand, by the time McClellan had been replaced and he was running on a platform or negotiated surrender, he might not have been quite as popular.