Jon's worried that his date will be ugly. That Jon has to keep monotonously repeating these platitudes tells me he doesn't believe them. It's funny, but that we "get" the joke means that deep down, none of us believe them either.
The Garfield canon seems to waver on how attractive Jon is supposed to be. I like a happy medium scenario, in which Jon is a regular-looking guy, but his bad taste, dullness, desperation and poor self-image make him unattractive. I like that idea because it makes him a better foil for Garfield, who is physically disgusting, has no interest in women, and is rude to all comers... but is the more charismatic and beloved of the duo.
Which brings up an important question: why is there not more Jon Arbuckle merchandise available?
Incidentally, in terms of collector's market, you can judge a book by it's cover and dust jacket, but you also have to open it and check the bind and page conditions.