What idea does the author develop regarding the alienation
Take a look at Boo Radley. He is at the bottom of the totem pole in Maycomb and the most misunderstood.
With Boo, it is hard for Scout to tell the difference between truth and gossip. There are so many stories about him that it is hard for the townspeople to accept that he could do nice things for other people. He also represents what is wrong with the town. He knows that he won't be accepted with the community at large and he would rather alienate himself than have the townspeople do it.
You could also look at Scout. Though she is the main character, you could certainly that - outside of Dill - she increasingly become alienated. Atticus at trial. Aunt Alexandra is too old and too lady like. Jem is growing up and trying to act like a young man. Scout, grew up as a tom boy, is now alienated from that youthful way of life and is being forced to conform to adulthood and acting like a proper young lady (as Alexandra would put it).