Jon needs to worry more because Garfield's going to... what? Kill him? That's a pretty good joke.
I'm not sure why Jon is addressing the fourth wall in the first place. It looks like if creepy-Garfield hadn't interrupted, Jon was about to hold forth on the many things he worries about. ... Which is another strip I would like to see.
Good: that the simple posture reversal between panels one and three creates rare visual balance in a Garfield and eliminates the need to have Jon say "on one hand..."
Bad: that we didn't get a whole week of Garfield trying to murder and/or eat Jon.