Jon dated The Creature from the Black Lagoon in high school. Perhaps he went to Gravedale High.
I'm not sure what this does to the reality of Garfield. There are pirate ghosts in the Halloween TV special, but I don't think that's canonical. Note to self: scour Garfield archives for other monsters. I guess it's not much weirder than his extended date with a woman raised by wolves. Or that he has a cat which stands on its hind legs and reads his yearbook with him. It's kind of sweet that Garfield is showing enough interest in Jon's adolescence to consent to looking at the book.
Jon's way of telling this story is confusing but enhances the punchline. He switches from wistful to disgust and disappointment in the space of .001 seconds. Also perfectly timed today is Garfield's reaction, his eyes popping open in panel 2, warning us that crazy shit is about to go down, as Jon placidly yammers on.
Also: Jon's yearbook is printed in color? We had no such yearbook budget at North High School, Des Moines.