Mayella has never been treated kindly and with respect in her life. When Atticus calls her Miss Mayella, a terms of respect in society, she thinks he's making fun of her. Remember, Mayella is uneducated and has lived her life surrounded by poverty, a drunken father who beats her, and loneliness. She would have no idea that Atticus is showing her respect.
When Mayella takes the stand in the Tom Robinson case, she is afraid of Atticus. She even stops answering his questions, and when the judge asks her why, she points to Atticus and says, “because of him." Mayella thinks Atticus is going to really go after her, and she is scared he will learn the truth. When Atticus calls her “m’am," she thinks Atticus is making fun of her. As a member of the community, no one has ever treated her with kindness or an ounce of respect. Most people shy away from the Ewells because of Bob’s behavior and their poverty. Because Mayella is devalued as a woman and person by her father and also by the community, she feels that Atticus is making fun of her. In fact, as readers, we know that Atticus respects everyone, and he is trying to show her some decency. This shows just how lonely and abused Mayella is: she doesn’t even expect to be treated with respect.
Mayella thinks that he is treating her abnormally, since she is not used to being respected by others. She is constantly scolded, lives with a drunk father, and she is uneducated.