The form of the Garfield Beware of Dog sign joke is normally that the sign seems absurdly specific or unlikely, then Garfield finds out it is accurate or ironic in a way that effects the degree to which one should be concerned about the dog's propensity to attack. This looks like it may be a different joke, but at heart it is not. The dog having a bag on its head does not render it unable to attack, though Garfield stands by comfortably, as if now that the sign's message is reconciled, he is safe from harm. The sign does not warn of traditional attack, but an assault on aesthetics: the dog's ugliness itself requires wariness. The bag on the dog's ugly face neutralizes the threat, so Garfield is "safe" and unharmed, though standing within inches of a growling dog twice his size.
But Garfield still favors us with his sidelong glance of revolted disappointment. Casual readers will probably interpret the expression as acknowledges of the outlandish image, or even the half-heartedness of the joke. I propose the strip is also about the aesthetic of Garfield itself. Garfield passes contentedly through the first panel, an uncluttered ideal Garfield landscape, with a mid-frame horizon line, and utter void of other details; most Dog Sign strips do not start with such an image. Piece by piece this ideal is cluttered with props debris and partially-coherent raw joke-material, first shocking, then disgusting Garfield as he forges further ahead into the mess and mystery. After the release of the punchline, Garfield seems less impressed by the resolution than repulsed by the effort of resolving the illegible. Garfield is happier with no one, and nothing else crowding and complicating the space without permission.