Atticus describes the use of the word "nigger" as something his children should avoid because it is "common." This word choice is interesting when considering the character of Atticus Finch, as it almost suggests something about the class of people who use the word "nigger." Perhaps only "common" people use that word, and Atticus has not raised his children, Scout and Jem, to be common, so therefore, they ought not use the word.
If this analysis is true, then Atticus Finch is a more complex character than his choices and behaviors indicate. He is a straightforward person with a clear moral system and a direct manner. He seems to see people as individuals, resisting stereotypes and other harmful and prejudiced ways of thinking. Atticus defends Tom Robinson because he thinks Tom is innocent, and he knows Tom is in a disadvantaged position as a poor, black man in the community. All of these descriptions characterize a good guy who uses his cleverness and integrity for good and not for evil.
But playing devil's advocate requires the reader to question Atticus and his use of the word "common." Isn't Atticus falling into a stereotype himself in this situation by suggesting that only people of a certain social demographic use the word "nigger"? Perhaps, in Maycomb, that is true, but the unfortunate reality is that people of all social classes use hateful words like "nigger." The reader can interpret Atticus here in this moment as an idealist who wants to believe that non-common people don't say "nigger." Alternately, we might interpret Atticus as a classist person who makes assumptions about people poorer and less educated than himself. Either way, Atticus remains a fascinating character in literature for all his truly human complications.