The big news is that the secret admirer note is apparently real! I was sure it would turn out to be a cruel Garfield hoax, but Jon's actually received a love letter.
Please note that Garfield's phrasing leaves room for the possibility that he does admire Jon and just isn't telling us.
If you wonder why a choice was made to have Jon spontaneously disappear for the last two panels, when the usual Garfield template would require him to stick around and be half-closed-eye-lidded and deflated by Garfield's withering insult, I propose: today Davis just gave the poor guy a break. Somewhere in another room, Jon is being happy with his note.
And that's the scene I like to imagine. Jon grinning and staring at that note for the rest of the day.