Yes, hoorah for those kind of girlfriends you see once a month. While I honestly don't think it's Jim Davis' intention for us to question the commitment of Jon and Liz's relationship, I do think it's funny that he's spent more strips talking about her than spending time with her.
Jon's excitement about the relatively distant future is not so outrageous. It is part of human nature to imagine the possible development of a fledgling relationship. Fantasizing that a couple might still be dating in five months is not necessarily ridiculous, and while Jon's bug-eyed expression is supposed to indicate that he has become overeager, it's not the soul of the joke. It is Garfield's inability and/or stubborn refusal to take interest in Jon's happiness. Jon often speaks to Garfield not just as a housemate, best friend, or confidant, but as if Garfield is part of his internal mental dialogue. The unrelenting cynicism sometimes functions as it's own celebration of bad behavior, sometimes as comic counterpoint to Odie's moronic glee, but against Jon it can run the gamut. Contained in Garfield's brief response is a healthy "reality check" for Jon, an observant lashing-out about the jealousy of friends towards a chum's new relationship, and perhaps best of all, unnecessary meanness for the sake of being the force of contrariness in this small universe.