There are two punchlines dueling for laughs but building on one another: Jon's indecisive dithering/enthusiasm for mundane chores and Garfield's disgusted evaluation of Jon's partying skills. Before Garfield's elaboration that chores and fun are unified in Jon's mind, Jon's dilemma is a basic one we all deal with daily. Objectively it is kind of funny that some nights you go out and goof off and some nights you stay home and do housework, and not only that but some nights you want to dust and vacuum. We're a social species, but a domestic animal as well, and maybe taking basic pleasure in activity like washing the dishes, scrubbing the bathtub and brushing our teeth keeps us from shooting ourselves in the mouth when we realize not-partying is really the majority of our time. It's a bleaker take on the idea behind Disney songs like "Whistle While You Work" and "A Spoonful of Sugar".
But even funnier than any of that are the first two panels. A man enthusiastically tells his cat he wants to party, as if the cat can do anything about it. And then he stares off over his shoulder, beaming, as if the party will spontaneously happen, or maybe sitting at the table with the cat was always going to be the extent of the party.