She is afraid of him because she doesnt want him to do what he did to her father during the trial (write her name on paper). She also beleives that he insults her by calling her "Miss", since no one has ever called her that before. That gives you a good idea on how she is treated by others.
Actually, Atticus doesnt mock Mayella. Mayella thinks Atticus is mocking her because he is caller her Ma'am and Miss Mayella, which she isnt used to.
The judge asks Mayella what she's scared of, and she points to Atticus
Mayella cries because she thinks Atticus will find out that her father really did it.
Bob Ewell, who is Mayella's father, beat her up
"Tempting a Negro"
Bob Ewell or Mayella
Mayella's reaction to Atticus' behavior in court is ironic because Atticus is the only person actually trying to defend her and seek justice for her, yet she is hostile towards him. Despite his efforts to help her, Mayella feels threatened by Atticus' questioning and sees him as an enemy due to her prejudiced views.
Atticus aims to show that Mayella is not a reliable witness by poking holes in her story and highlighting inconsistencies. He tries to establish that Mayella's accusations against Tom Robinson are a result of her own personal issues and not based on factual events. Atticus demonstrates that her testimony lacks credibility, bringing into question the integrity of the case against Tom.
When Mayella is on the stand the question Atticus asks that makes her furious is "How?" She has just described how Tom Robinson choked and beat her, and Atticus asks Tom to stand up. This draws the jury's attention to the fact that Tom's left arm is useless, making it highly unlikely he was Mayella's attacker. When Tom stands, Atticus asks Mayella if Tom is the man who attacked. When she responds that he is, Atticus asks, "How?"
Mayella Ewell was surprised and taken aback by Atticus's politeness during the trial. She had likely never encountered someone treating her with respect and courtesy before, which made her uncomfortable and defensive.
Atticus wanted the jury to have a picture of Mayella's life because he was trying to get everyone to see that she has to do all the chores, and she like the "outcast" of the family. He was also trying to show that Bob Ewell gets mad and drunk a lot at Mayella. And that is how she gets beat, and raped.
In those chapters, Mayella Ewell's thoughts about Atticus Finch are complex. While she falsely accuses him of assaulting her in court, it is implied that she may feel conflicted about him due to his respectful demeanor towards her during the trial. Mayella comes from a difficult home situation and may have mixed feelings about Atticus, who treats both her and her father, Bob Ewell, with dignity and respect.