Panel 1: What is Jon doing?
Jon is dressed up in a typically pattern-blind checkered suit and polka dot bow-tie combo, and standing around by his table, staring off into space.
Panel 2: What is Garfield doing?
Garfield has inverted a paper bag over his head as a sort of improvised mask. Two tiny holes have been cut in the bag to facilitate Garfield's vision. These holes are not nearly large enough to accommodate the Garfield's bulbous eyes, currently estimated at four to six inches in height.
Panel 3: Mysteries of meaning.
An ensemble like this usually signals that Jon is going on a date, and thus leaving the house. Garfield may be implying that Jon's attire will cause pointing and stares when the man eventually goes out, but at present they are just standing around at the table where there is no one to see either one of them. Jon takes Garfield's meaning well enough, but refuses to listen, despite decades of criticism of similar outfits and from sources independent of Garfield's skewed opinions.
Garfield makes the complicated assertion that "the bag doesn't lie." In one possible sense, this means that as one creature on this planet is disguising his identity lest he suffer humiliation due to association with Jon's clothing, then Jon is, indeed, embarrassing to be seen with. The very presence of Garfield's point of view negates Jon's emphatic statement of self-worth.
The stranger innuendo is that the bag speaks The Truth, chooses its wearer because it must be worn. Between the warring forces of Jon's clashing fabrics and The Bag, Garfield is powerless. His paw is forced, and he is crowned with The Bag through necessity, not to editorialize. Thus Garfield insinuates that his personal taste is equivalent to an objective fact.
Which is more embarrassing, a badly dressed man or a cat walking around with a bag on its head? Which is more endearing?