Actually Nixon's unattractiveness wasn't so important as the perception that he was an unprincipled, ruthless opportunist. This perception long predated Watergate. The nickname "Tricky Dick" originated in 1950 when Nixon used a cruel smear campaign to win election to the US Senate. This negative perception of Nixon, which turned out to be largely correct, haunted him in 1960, 1962, and 1968.
In terms of PR trumping substance, this was true to a certain extent in 1960. But ultimately Nixon's critics were proved right by his handling of Vietnam and of course the Watergate scandal. In terms of actual substance, Nixon is far from impressive as an American President.
In other words, he was a politician! From what I read, the 1950 campaign was particularly dirty on all sides with Tricky Dick having been coined by one of Nixon's opponents; let's face it...it's good and catchy!
As for substance, he is seen as having ended the Vietnam War and splitting China from the USSR. Of course, these are foreign policy results but they are the ones which count for other countries.