I guess the joke is that even though people are in danger, Garfield can't decide if he should prevent their deaths or eat a donut.
It kind of falls apart on realizing that Garfield is just pretending to be a superhero, and no citizens are really in distress - i.e., there are no stakes to his being distracted by a donut.
Funniest part by a mile: the expression of sheer horror in panel two, as Garfield sees a donut and knows he shouldn't eat it, knows the repercussions, but knows that he can't stop himself. He knows his failings, his lack of willpower, knows he is controlled by destructive drives. Garfield doesn't want to eat the donut, and knows he will eat it anyway. Even in his fantasy world, Garfield cannot saddle his unhealthy food addiction to save a human life.
