I'm assuming Garfield is comparing Shecky Green's outdated comic stylings to Jon's squaresville lingo. The "thing" is, Shecky is funny. So maybe Garfield is contrasting them. Or perhaps I'm getting oversensitive as someone who actually does say "How's tricks?"
So this strip has some problems for me, including the presumption of the core audience's familiarity with moldy nightclub comics. I do like that Jon thinks saying hello and asking how someone is doing is an opportunity for wit.
I never before considered how strange it is that when the design change was made to turn Garfield's paws into hands (there are no foot pads, and he has opposable thumbs. Uck... ), a decision was made to continue drawing the mutant body parts as paws unless being used as hands. So in panels 1 and 2 nothing seems to be amiss, until: BLARG! CAT HAND. Cope with it, America!