It reveals that Atticus is above the accepted social codes of Maycomb. There now have been several instances where Atticus has been references like this. Scout beat Cecil Jacobs up for a similar references. Scout lets Francis have it at Christmas time when Francis called Atticus that (no doubt hearing it from Aunt Alexandra). And now Miss Dubose adds to it.
Maycomb is deeply racist and set in those racist ways, so they feel Atticus should do very little to help Tom. Furthermore, even attempting to help him is seen as unimaginable. Yet, Atticus doesn't accept those beliefs.
All their lives Atticus has told the children to have empathy and compassion. How could he not accept this burden? This is scene when he explains to Scout that "N-lover is just one of those terms that don't mean anything -- like snot nose. It's hard to explain -- ignorant trashy people use it when they think somebody's favoring Negroes over and above themselves" (108). This is keeping in line with Atticus's ability to empathize with others. He most certainly is putting Tom's welfare over his own. Just look at the repercussions that affect Atticus and his family after the trial.
Lee gives us insight into the depth of Atticus' tolerance and his ability to maintain his own dignity and approach other people, even those who attack him, with understanding. He says, "it's never an insult to be called what someone thinks is a bad name. It just shows you how poor that person is, it doesn't hurt you". He says in actuality he really is a nigger-lover because he always does his best to love everybody, no matter what their race, and he counsels Scout not to let people who He says that when all is said and done, they do it because they have serious problems of their own (Chapter 11).