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Mayella feels that Atticus is making fun of her. Although Mayella is white, she is a Ewell and is considered in very low esteem by the community. Nobody has treated her with that kind of respect. Mayella doesn't quite know how to handle being addressed in a respectful manner.
Mayella has very little understanding of polite, ordinary society and is quite limited in her understanding of relationships. We can see this in the way she has tried to engineer a 'relationship' with Tom Robinson. When Atticus addresses her with such courtesy, Mayella can only interpret his intention as cruel and derogatory - it is how she is treated by every other white man, after all.
It's part of Atticus' job as a lawyer!