aaaagh
Explanation
The Joke
Superman is flying overhead, but instead of his usual confident demeanor, he is screaming in terror -- "AAAAAGH! AAAAAAGH! AAAAAGH!" -- as he soars above the city. Below, two criminals stand in an alley, and one of them announces that he is giving up his life of crime because he "can't take this anymore."
The caption at the bottom delivers the punchline: "Crime rates nosedived shortly after Superman developed his fear of heights." The joke inverts the typical Superman narrative. Normally, Superman deters crime through strength and authority. Here, he deters crime even more effectively by being terrified -- his constant screaming while flying is so annoying and unsettling that criminals simply cannot stand being around it.
The Humor
The humor works on multiple levels. First, there is the absurdity of giving Superman -- a being who flies constantly -- a fear of heights. Second, the comic subverts the expectation that a superhero needs to be competent and intimidating to fight crime. Instead, Superman accidentally becomes more effective at crime prevention through sheer obnoxiousness. The criminals are not afraid of being caught; they are just tired of listening to the screaming. It is a commentary on how sometimes an unintended side effect can be more powerful than the intended action.