Mayella Ewell is, at least to some extent, frightened of Atticus Finch. Although she is probably being rather melodramatic, she bursts into tears at the beginning of her session as witness; despite her dishonesty regarding what truly happened in relation to her interaction with Tom Robinson, she is a teenager who has been placed in a frightening and uncomfortable situation.
It is important to consider her upbringing: Mayella grew up in a home where she was shown no love and seen as "white trash" by those around her. She has obviously been given no reason to respect herself and is extremely defensive when she misunderstands Atticus's questioning and believes that he is mocking her; being made fun of would obviously have been no new experience to her. Mayella is unaccustomed to being the recipient of kindness, respect, or courtesy of any kind, so she believes that Atticus is making fun of her when he addresses her with politeness.
As he did with father Bob Ewell, Atticus Finch plays hard ball when Mayella Ewell takes the stand during the Tom Robinson trial of To Kill a Mockingbird. Atticus does all he can to discredit Mayella: He questions her education, her social standing, and her relationship with her father. He casts doubt on nearly every aspect of her story. Mayella does not take kindly to the interrogation. She accuses Atticus of trying to scare her and mock her.
Mayella sniffed wrathfully and looked at Atticus. "I'll answer any question you got--get me up here an' mock me, will you?
After Atticus suggests that Mayella initiated the physical contact with Tom, she explodes.
"... if you fine fancy gentlemen don't wanta do nothin' about it then you're all yellow stinkin' cowards... the lot of you. Your fancy airs don't come to nothin'--you ma'amin' and Miss Mayellerin' don't come to nothin', Mr. Finch."