As it turns out, Garfield is brilliant

[Dark Jaguar]

I still don't see what you mean by "Jon makes no sense". He's losing his mind and talking to himself out of the sheer boredom and hopelessness of his quiet suburban life. That said, everything he says without the cat makes perfect sense in that context.

Think of it, a man sitting at a counter looking back and forth and suddenly saying "we have the best maintained toaster on the block", as though he's just been there doing nothing all day and had to say one thing to justify his existance, before he notices gas prices at $6 and puts his head in an oven.