Why does Mayella think that Atticus is mocking her in To Kill a Mockingbird ?

Mayella Ewell believes that Atticus is "sassing" her when he calls her "ma'am" and "Miss Mayella."

Mayella tells Judge Taylor that Atticus is mocking her when he has actually addressed her in terms of politeness. The judge informs Mayella that Mr. Finch is not making fun of her. Then, the judge asks her, "What's the matter with you?" because he is amazed that she knows nothing about manners or the diction of polite people.
Hearing all this, Scout wonders if Mayella has never been called "Miss" before, and this is the reason that she would take offense with common courtesy. Scout asks herself, "What on earth was her life like?"

After extracting from Mayella what her life is like as the children fend for themselves because the relief check is not enough, or her father squanders it, Atticus asks her if she has any friends. Mayella repeats the words questioningly. Atticus responds again,

"Yes, don't you know anyone near your age, or older, or younger? Boys and girls? Just ordinary friends?"

Something about this question enrages Mayella. "You makin' fun o'me again, Mr. Finch?" Atticus lets this question be the answer. Then, he asks her about what happened with Tom Robinson. It is obvious that Mayella tries to remember what to say. When Atticus asks her if Tom hit her, Mayella first says that she cannot remember, then she says, "I mean yes I do, he hit me." And, she begins crying. When Judge Taylor scolds her, "Don't you cry, young woman--" she cries more.
Again Mayella "sniffed wrathfully" and looks at Atticus, saying, "I'll answer any question you got--get me up her an' mock me , will you? I'll answer any question you got--"

While Mayella confuses Atticus's professional politeness as mocking her, nevertheless, she is aware that Atticus asks her such specific and numerous questions because he hopes to discredit her testimony. She is uncomfortable because she has never been on a witness stand; however, her real discomfiture comes from the fact that she is well aware that she is falsifying her testimony.