Atticus believes that all people deserve dignity and respect, regardless of their status.
Atticus treats people who have relatively low status with dignity and respect. The Radleys, the Cunninghams, and the Ewells are all of low status for one reason or another. The Radleys are normal people, but because of Boo Radley they are mocked or looked down upon by the rest of Maycomb. People gossip about them, but Atticus never takes part in the gossip.
He said Atticus never talked much about the Radleys: when Jem would question him Atticus's only answer was for him to mind his own business and let the Radleys mind theirs, they had a right to … (Ch. 1)
This is one of the reasons why he does not want his children enacting plays about Boo Radley in the neighborhood. He thinks that they are mocking Boo. The children may begin by considering Boo just the neighborhood monster, but they soon come to consider him a friend. When Atticus realizes that, he respects them for it.
Atticus also treats the Cunninghams with respect, even though they are poor.
One morning Jem and I found a load of stovewood in the back yard. Later, a sack of hickory nuts appeared on the back steps. With Christmas came a crate of smilax and holly. That spring when we found a crokersack full of turnip greens, Atticus said Mr. Cunningham had more than paid him. (Ch. 2)
He is able to talk to Walter like he is a grown man, and treat him like a man of dignity, because he can see beyond the poor farmer. He is also able to accept payment in chickens and stovewood. Atticus is able to see people for who they are, and does not subscribe to stereotypes.
Scout tells her teacher, Miss Caroline, “The Cunninghams never took anything they can't pay back” (Ch. 3). This is her father talking. She learned this from him. He has taught her to respect them and their ways. Even though she still looks down on Walter’s strange eating habits, and has to get a lesson in respecting company from Calpurnia, Atticus has clearly tried to teach his children to respect people’s values.
Atticus also tries to explain this to her when it comes to the Ewells, a family who take having their own values outside of society to extremes. Considering the trouble that the Ewell family is making for Atticus, you can understand why he would be frustrated with them. Atticus explains that the Ewells are different, and that law makes exceptions for them. He is not very kind in his explanations of them.
Atticus said the Ewells had been the disgrace of Maycomb for three generations. None of them had done an honest day's work in his recollection. He said that some Christmas, when he was getting rid of the tree, he would take me with him and show me where and how they lived. They were people, but they lived like animals. (Ch. 3)
While this is not a nice description, it is more factual than judgmental. Atticus is frustrated with the Ewell’s lack of education. He notes that they can go to school, “when they show the faintest symptom of wanting an education,” but they don’t (Ch. 3).
You will notice that during the trial, he does treat Mayella Ewell politely and respectfully. She has never been treated politely and respectfully in her life, so she thinks that he is sassing her. Scout feels empathy for Mayella too, realizing that she is the one Ewell who wanted more from life.
Atticus is a man who believes that all people should be treated respectfully. He treats the poor, the maligned, and the ignorant with respect. This is a lesson that he passes on to his children, and one that Scout learns throughout the book. It is why Scout and Jem are so concerned with befriending Boo Radley. They see him not as the town monster, but as an outcast in need of a friend, and decide that if they treat him with respect, he can have a chance to be one.