Why does mayella say she is afraid of atticus when she first takes the stand? why does she think atticus is mocking her as her testimony proceeds?

She's afraid of how easily he can manipulate his words to make anyone slip revealing information. She's afraid of his art and craft at what he does.

To answer the second part, she got defensive when Atticus calls her "Miss" and "Ma'am" because normally, they are not at a respectable enough social status to be called such.