I disagree. Reagan in 1980 was one of the smartest campaigns I've seen. It was also helped by Carter's real incompetance in campaigning, something which showed itself towards the end of 1976.
You say that Carter's approval ratings were in the mid-30s, so Reagan should have beaten him by 20%. That is clearly wrong. Even when approval ratings are low for incumbents, there are generally always a certain amount who will vote for their candidate simply because of party ID. For example, Bush 41 was mired in the mid to upper 30s in approval rating throughout most of the 1992 campaign and still only lost by 5.5%.
If you want another subpar campaign that won, the best example IMO is Nixon 1968.
Nixon always reportedly said that if the election had been held a week later Humphrey would have won. Humphrey's began very badly but he picked up towards the end and lost very narrowly; in fact wasn't 1968 closer than 1960? I think that haunted Nixon which was why he was determined to win decisively in 1972.
