What do you learn from Bob Ewell's evidence?Is it that Mayella was abused and harassed?

Probably the most important thing we learn is that Ewell is left-handed. Whoever hit Mayella Ewell on the night of the alleged rape was also left handed. The bruising on her face, especially on her right eye means that someone was leading with their left hand. Tom Robinson has no use of his left hand because he got it caught in a cotton gin when he was a boy. Therefore, the most likely person who hit Mayella was her own father.