What is ironic about Steve's imprisonment in Monster?

It is ironic that Steve Harmon is in jail because he is found not guilty in the robbery and murder of Aguinaldo Nesbitt. In prison, he is surrounded by convicts and inmates that are guilty of committing various crimes. These criminals are essentially bad, violent people who made conscious decisions to participate in crimes. Steve Harmon is not intrinsically like them and has a conscience. He views himself as a good person with morals. Even though Steve knows that he is inherently different from the other prisoners, the inmates, guards, jury members, and attorneys perceive him to be the same. It is ironic that Steve Harmon is categorized as a criminal like the other inmates, despite the fact that he is a good kid who is innocent. Steve does not deserve to be in prison because he is not guilty of robbing or murdering Nesbitt. 

What is ironic about Steve's current situation in Monster?

In his recording of his thoughts, Steve reveals more of himself than he intends, lending irony to what he states. In this way, without realizing it, he answers his own question about whether he is the "monster" that the prosecutor accuses him of being. For example, when he gazes at himself in the mirror of his cell that has other names scratched into it, he remarks that his reflection "does not look like me." Thus. the mirror symbolizes the image of himself that Steve presents to others, and not what he knows himself to be.

In another instance, recorded in Journal #1, Steve hears the testimony of the second witness, Sal Zinzi, an inmate of Rikers, who is called by the State Assistant District Attorney. Zinzi, who has been convicted of receiving stolen goods, testifies that he learned of the robbery of Mr. Nesbitt's store. Suspecting that Zinzi now offers testimony in order to cut a deal so that he can be released early because other inmates have threatened to rape him, Ms. Petrocelli asks Zinzi if he knows when he is lying and when he is telling the truth; Zinzi says he does, but the prosecutor responds to his answer,

"Let me get this straight....You'd buy stolen goods for profit, rat on somebody to save your own hide, but you're too good to lie. Is that right?" 

As he listens to this exchange between Petrocelli and Zinzi, Steve recalls a similar behavior on his part. In this "Flashback," he is twelve and with his friend Tony in the neighborhood park. There, Tony brags of his pitching ability, and, to demonstrate, he picks up a rock and throws it; however he throws poorly. Boasting that he can do better, Steve picks up another rock and pitches it, but his bad throw hits a woman. When the man with her demands who has thrown the rock, Steve yells, "Tony, run!" Naturally, the man assumes that Tony is the one who has thrown this rock, so he punches Tony. Fortunately, the woman convinces the man to stop and just leave with her. While the boys sit on the ground,

TONY  I didn't throw that rock. You threw it.
STEVE I didn't say you threw it. I just said "run." You should've run.

Having heard Zinzi words, Steve now realizes that he was motivated selfishly to save himself that day in the park, "ratting" on his friend to protect himself from the man, just like Zinzi.

Later, when Steve is on the stand, he lies in order to convince others that he was not involved in the robbery. But, Steve knows that he gave no signal after coming out of the store because he wanted to remove himself from the situation just as he had done with Tony in the park.

Ironically, however, Steve cannot understand that he possesses sociopathic tendencies. This is what his attorney knows, and it is what his father wonders about:

My father is no longer sure who I am...He wonders what else he doesn't know.....
That is why I take the film of myself. I want to know who I am. I want to look for one true image.

At the end of his narrative Steve looks into the camera just as in his cell he has looked into the mirror scratched with many names. But, as Jonathan Swift once observed, "There's none so blind as they [who] will not see."