The staging here is Garfield 101: an outlandish sight gag is the punchline to a story about bad behavior, but remains off-panel, and is presented to us only by the cast's shocked and/or laconic reaction. In this case, the un-sight gag is the destroyed sofa and the pet sitter's weightlifting performance. One school of cartooning understands this technique as a gyp, and the Garfield reader knows that half the joke is that we don't see the joke. To untangle this reasoning a bit:
A) The spare staging of a Garfield daily is nearly always about paring away unnecessary information and stimulus, both in the name of clean, minimalist gag writing, and depriving the audience of some traditional form of pleasure and payoff.
i) In some regards, this technique strips potential joy and liveliness out of the strip, but matches the Garfield tone, worldview and characters' experiences. The reader is free to find this cynical, frustrating, lazy, or ingenious as she sees fit: no answer is wrong, and all are thoroughly appropriate for the strip.
ii) On the other hand, the non-traditional staging helps Garfield avoid certain easy gag-strip pacing clichés. Today's episode is perhaps not a prime example, and the off-frame outrageous incident style carries its own historical baggage as well, but it is the less common option for funny animal cartooning. In a way, Garfield makes you work for the gag a little harder.
B) Relegating complex sight gags to off-frame serves several practical purposes regarding the cartoonist's physical labor.
i) Given Davis's big-shape, bubbly style, there is no logical way to stage the couch-lifting, including Greta, the couch, and any reacting witnesses in a way that would read. The daily strip panel is simply too small.
ii) The trashed couch would be hard to draw. Especially in Garfield style.
C) Seeing the couch would not actually be funny in and of itself. Nor would seeing the terrifying sight of Greta mangling the couch. However...
D) This isn't about the couch, or Greta's uncouthness, or even the intrusion of hyper-masculine behavior into the all but degendered Arbuckle household. This is about Jon, Garfield and Odie's reactions in the aftermath of Greta's visit. There might have been a funny freak-out reaction moment in Jon finding the trashed sofa, but we're in some undefined period after that, and he's had time to readjust. Readjustment, a return to normal in Garfield is usually a rapid slide back into slight disappointment and weariness. The key here is that Jon doesn't even raise his eyelids in surprise. This is how things go in this strip.
E) For all the pets-in-panic fuel the strip got out of Greta for a few days -- she posed a physical threat, claimed she would impose discipline, and cast a strange air of gender confusion over the house -- in the end nothing came of it. In the only glimpse we had of Greta interacting with Garfield and Odie, she was letting them sit on the couch and watch TV with her, which is business as usual. In Garfield and Odie's perception, Greta's only crime was making them uncomfortable by being unattractive and defeminized.
So given that i) it's unlikely that if the sofa were clean and jerked in the manner Odie indicates that it would be "bent", and ii) Greta, established as obsessed with discipline, would destroy a client's property and leave with no explanation, we are led to wonder:
Did Garfield and Odie somehow bend the couch, and drive Greta out of the house, then blame it on the pet sitter to avoid Jon hiring her again? In the end, Jon's home would've sustained less damage had he left Garfield and Odie with run of the house, so he's screwed either way. Which is, of course, the way things go in Garfield.
Happy New Year, sucker.