Since the Ugly Date Outfit is a regular staple of the strip, we may assume Garfield is not laughing at Jon's clothes.
Garfield is laughing because of the serenity on Jon's face as he sits at the table contemplating whatever doomed social outing has spurred him to dress up; Garfield knows nothing about the situation save that if Jon is going out, this Saturday night will end badly. He is, in a way, preempting the punchline of the date-gone-awry strip. Why bother telling the joke, anymore?, asks today's strip: just show Jon ready for a date, and have Garfield laugh at him.
Or maybe Garfield is laughing because if he opens his mouth wide enough while standing on the table, he looks an inch taller than Jon.
And maybe Jon looks shocked in panel two because of how small Garfield's torso has been drawn.