Every panel is funny today. Every beat is a perfect measure, musical in precision.
"Uh, Liz?" is such a shitty opening for a come-on; it's unusual because it's real and recognizable, unlike Jon's usual gooniness which is an over-the-top placeholder for our own awkwardness.
"Yes?": Really, it's Liz's expression. "Uh, Liz?" is only two words, but they're so exact they can only mean one thing: you're gonna get totally, earnestly hit-upon. For whatever motivations of her own, Liz is happy about this, and that promise is what will fuel this joke. Because sometimes all you have to say is "Uh, Liz?", witness the scrunched expression of panic and cornered dismay. And that expression, my dear ones, is not funny in the least.
Jon, Panel 2: Jon and Garfield's poses are unwavering today, but the shades of meaning are like a rainbow. Well, a three-color rainbow. Here the eager confidence chills in Jon's chest, and though they're the same circles and dots, I swear I can see them glaze over with sick green fear.
Liz, Panel 2: And yes, Liz's face registers the shift between thinking Jon is sweet and deeply weird, but the sudden retraction of her hand from Garfield's back seals the deal: she wants so little to do with Jon that even touching his cat feels wrong.
Liz, Panel 3: There's not much space in Garfield for timing, but the disappearance of Liz is sudden enough for a laugh, and evocative. Did she run? No, she backed away slowly.
"I choked, didn't I?": Could be Jon blacked out in panel two. Could be he is in denial. Could be he wants confirmation because he really has no idea if Liz's terrified escape means he succeeded or not. Could be Jon doesn't even know what reaction he wanted. I'm happy with any of the above, because the laugh-aloud is Jon's frozen grimace and wide-eyed terror at what he's done. Hooray!
Garfield's punchline: Yeah, I laughed again.