If there's a reason why Gary Coleman decided to pose for his mugshot looking like this, I don't know what it would be.
In the first place, it could have been snapped when he was still upset. That makes the most sense to me. Shame on you, jail photographer. You should have told Gary a joke and you should have had him take off that sweatshirt.
In the second place, Gary Coleman has never really had a leg up on manipulating the media. He's always been behind that power curve.
Third, absent a lawyer or advisor or someone to mentor him, there's no one to tell him how to pose for a mugshot when you're a celebrity. He appears to still have an agent, if you read the story that I have linked above. Fat lot of good it is doing him.
This is how you pose for a mugshot:
That's how you do it. You smile. You smile like the dickens. You make it look like there's been a happy accident, and you're the happy warrior, slap-happily obliging Johnny law. Make it look like you just said gosh-darn out loud and offered someone some pie.