With a narrator as obsessed with his hair as Ponyboy Curtis is, it's no surprise that that the other characters' looks are also important to him. Clothing and hairstyles might seem like superficial markers, but they're also the means by which people express their public identities. Pony and his gang don't have the cash for designer clothes, but they still manage to develop a distinctive style, which identifies them as Greasers. For much of the story, Pony wishes he had the money to dress more fashionably like the Socials. But he comes to learn that expensive clothes and cars aside, the Socials have problems just like the Greasers do.
Pony also spends a lot of time on faces. Dallas Winston has "dangerous" and "hardened" written all over his face. And you can take one look at Johnny Cade and see in his eyes that he's a victimized, hungry, frightened kid. Of course, both Johnny and Dallas are also much more than this, as Pony comes to see. Johnny is also brave-hearted and Dallas is full of love, if only for Johnny. Pony learns he's misread both of his brothers too, because he hasn't been looking at life from their perspectives and has been fooled by their appearances.
Appearances in The Outsiders tell only half the story; ultimately, actions speak louder than appearances.
Ponyboy writes his story to encourage others to stop judging Greasers by their appearances.