Saying that every president, especially Republicans, were worse than Nixon, and the scandals were worse than Watergate, does two things for John Dean.
(1) It makes him feel better about himself and his role in Watergate, to which he went to prison for. After all, if everyone's worse, he can't be all that bad, right?
(2) It's a living. He's sold books and gets paid for TV appearances, spouting off on the subject. Since he's been to prison and is disbarred, he has to make a living somehow, enough of one to keep up the country club cocktail circuit lifestyle.
He is a Fraud.