In Lee's To Kill a Mockingbird, what is made clear during Mayella Ewell's testimony?

In Lee's To Kill a Mockingbird, Mayella Ewell's testimony in court is found in chapter 18. Based on Mayella's performance on the witness stand, it becomes clear that she is a melodramatic, uneducated, socially ignorant, and abused liar. Most importantly, though, because of Tom Robinson's crippled left arm, there is no way that he could have blackened her right eye. For example, after Mayella claims that Tom blackened her right eye, Atticus has Tom stand up to show her and the courtroom that his left arm is crippled. Scout describes this revelatory moment in the following passage:

"Tom Robinson's powerful shoulders rippled under his thin shirt. He rose to his feet and stood with his right hand on the back of his chair. He looked oddly off balance, but it was not from the way he was standing. His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him" (186).

Tom's crippled arm is a revelation to Scout and Jem, as well as to the reader, but not to Mayella. What becomes clear to the children and the reader is that if Tom Robinson is physically unable to hit someone with his left arm, then Mayella could not have sustained an injury on the right side of her face; therefore, what Mayella claims to be true is a lie. And if Mayella is lying about how her eye was blackened, then she is most likely lying about being raped as well. Furthermore, during Bob Ewell's testimony, Atticus proves that Bob is left-handed by asking him to write his name down on paper in front of everyone. An educated member of the jury should be able to deduce that the physical evidence proves Tom could not have hit Mayella with his left hand, but her father certainly could have.