Panel 1: Damn it, Jon, seriously, you left a plate of donuts out on the table unguarded? You expect to be eating any donuts today? Though he is not in it, this panel tells us more about Jon than Garfield; about his bottomless capacity for trust, and refusal to admit how his friend treats him. Fall guys are funniest when they set up their own fall.
Panel 2: Well. At least Jon was only expecting "a donut" out of an entire plate of sinkers. When a man's dreams are so small... it just makes them easier to crush.
I haven't anything insightful to add about it, but Garfield standing with hands on hips and expression of satisfaction at a mouthful of fried dough gave me a laughing cramp.
Panel 3: The giant cat tongue is a good sight gag, offering saliva-drenched food a good gross-out gag, and the conciliatory gesture Garfield knows Jon would never accept a sharp character note about remorselessness and insincerity. Note also: It is impossible to know how much time elapses between Panels 1 and 2, which is the secret power of the punchline. In Panel 2 it looks for all the world like Garfield is chewing the donuts. Surprise! This begs another question. If all six donuts visible on the plate are accounted for on his tongue, plus another presumably buried in the stack, plus another now mushed into his cheeks, where in his anatomy was Garfield concealing the 8+ donuts? And where might we get a donut with pale blue frosting?
Verdict: Garfield donut jokes are hilarious.