Mayella's testimony implies that she is reciting what she has been told to say, while some of the things she says are facts and others are simply made up.
Scout narrates that "[A]pparently Mayella's recital had given her confidence" indicating that her testimony has been rehearsed; also she adds that Mayella's account seemed "stealthy,...like a steady-eyed cat with a switchy tail."
Certainly, there are inconsistencies with Mayella's testimony. For instance, she tells Atticus that Tom walks past her house every day, but the time that she has been supposedly raped was the first time she has invited him into the yard. (When Atticus asks her if it were the first time she has asked him into the yard, Mayella "jumps slightly at the question.") Then, she adds that she may have asked him previously to do some odd jobs.
Further, Mayella testifies that Tom has grabbed her around the neck and struck her and "chunked me on the floor an' choked me'n took advantage of me." Then, he supposedly choked her and beat her on the face; however, when Atticus has Tom Robinson stand up, and his left arm is withered and shrunken and obviously useless, her credibility again comes into question.
Watching Mayella wringing her handkerchief and listening to her contradictory testimony, Scout wonders if Mayella "has good sense." Mayella claims that her father has never touched her, but she says that he was standing over her "hollerin'." She says that her father has never laid a hand on her, but before this, she said that what happens between her and her father does not count. And, of course, her testimony against poor Tom Robinson is clearly questionable.