S.E. Hinton does not use the word "outsider" in the novel. Obviously, she wants us to ponder who the outsiders are. Who are "the outsiders"?
I think that Hinton is deliberate in suggesting that "the outsiders" are the Greasers. Hinton's construction of "the outsiders" is a social reality in which there are distinctions. She wishes to give voice to those who are marginalized in any social condition. In this case, it is the Greasers who are marginalized. The Socs are in the position of social power. If anything, they are "the insiders" who are able to "jump greasers and wreck houses and throw beer blasts for kicks, and get editorials in the paper for being a public disgrace one day and an asset to society the next."
Economics is a part of this. The Socs are financially well off and better than the Greasers, who lack the social standing. The issue of social division is one that does not create two equal groups, but rather creates a condition of "separate, but unequal." The "outsiders" that the Greasers are continually denies them opportunity and a sense of totality. Dally's self- destruction is a response to what it means to be a perpetual outsider. In presenting life as such for the Greasers, Hinton makes clear what group she feels "the outsiders" represent.