Holden's red hunting hat could indeed be described as representing his isolation from society. You could argue that it is a manifestation of his individuality as well as his rejection from society.
I would probably pick a key scene from the novel that has clear links to him either feeling a sense of belonging (try the final scene where he's with his sister in the park) and then find a scene you can contrast it with earlier in the novel that portrays his clear disconnection with the society he sees as 'phony'.