Now that Garfield has wadded up the spaghetti and put it in his armpit, Jon is not interested in eating it. On first scan, the gag is that the cat has swiped his master's dinner yet again. Garfield's inventive ruse, disguising the pasta as a ball of yarn, takes advantage of both the feline propensity for yarn-assisted frolics and the strip's art style which does not allow the eye to readily distinguish between food and fabric.
And that joke is there. Garfield steals this food although Jon's phrasing probably implies that Garfield was going to get a portion of spaghetti anyway. If his motivation were just to eat unseasoned, plain spaghetti, Garfield could have completed his mission alone in the kitchen. If he feared Jon's return to the kitchen, Garfield could have concealed the food as he spirited it away to a safe location. Garfield goes through unnecessary labor and trickery to dupe Jon for a matter of seconds.
Because ultimately this is not about Garfield's appetite for food. Garfield wants Jon to know that dinner is ruined. He wants Jon to know that he could have stopped it. He wants that middle panel, that moment where Jon realizes what is happening, what it means, that the man is a fool and the cat is triumphant, malicious, and a complete prick. And that, ladies and Nermals, is another sort of appetite.
And: I don't want to turn into one of those guys, but the missing hyphen in Jon's first word balloon makes my palms itch. As long as I'm being one of those guys anyway, that looks more like a fettuccine or tagliatelle.