With all due respect to the poster and to the one person answering so far, I don't see how anything in Mayella's testimony proves that she's lying. What we have when we read the trial scene in To Kill a Mockingbird is a set of conflicting first-hand accounts. The fact that Tom Robinson's account contradicts Mayella's does not prove the she is lying and he is not. The novel sets us up to side with Tom Robinson; the Ewells have consistently been presented to us in the narrative as despicable humans and the Robinsons as upstanding citizens.
My point is not that the two of you are wrong! Not at all! I simply wish to point out a very clear bias in the narrative that we, as readers, can resist. There's a very similar bias in the discussion of birds, for example; "you can shoot all the blue jays you want," we're famously told, "but remember it's a sin to kill a mockingbird." In reality (ask a real bird expert!), mockingbirds can be extremely aggressive and territorial just as blue jays have their own important ecological niche. Reading as a "resistant reader" is a good way for us to move past the same, standard interpretations of a literary work.
To me, Mayella may or may not be lying, but she's certainly very much aware that she's in a sticky situation and asks repeatedly for clarifications to Atticus' answers. (I've always wondered, for that matter, about her comments about her father and Atticus' follow-up questions. Does he avoid bringing up questions about incest or domestic abuse because he finds them untasteful, or does he simply not see that as a possibility?)