The awesomeness of Jon's ineptness with women is that he is not nerdy and shy around them, but nerdy and far too confident. Nebbishness wears thin for me, misguided bravado does not.
Yes, yes, Jon's pet name for Liz is weird and yes, would've been funnier if it were "Snooky-Wookiee", but I think we all know the meat here is in panel three. A lot of people would've had the punchline be that the overweight ugly receptionist gets excited by Jon's come-on, but Jim Davis allows no character to feel loved: she's horrified.
RE: Panel Three
-Surely I am not the only person who at first glance before reading the word balloons thought that in panel three Jon morphed into a fat lady named Nel? The layout of all three panels is so identical that there's no reason to suddenly think we've "cut" to a different building... especially since that happens about twice a year in Garfield. This might've been remedied by, say, putting Nel's computer on the opposite side of the frame, or her body in a position that doesn't exactly mirror Jon's. I offer these solutions though I don't want it remedied.
-As do all good veterinary receptionists, Nel keeps a green condom in her pencil cup.
-And a tiny blue Odie serves as Salacious Crumb to Nel's slug-bodied, earless Jabba.