In Chapter 20 of To Kill A Mockingbird, Mayella's offense is that she has  tempted a black man by kissing him. She tried to hide that offense by saying Tom Robinson attacked her. Of course, this is a lie. Mayella was attacked but not by Tom Robinson. Mayella is at fault. She tempted a Negro man. Then she blames him for her attack. Mayella lies to the court to get rid of her own guilt:

"What was the evidence of her offense? Tom Robinson, a human being. She must put Tom Robinson away from her. Torn Robinson was her daily reminder of what she did. What did she do? She tempted a Negro.

Mayella kissed a black man. She tempted him. Then her father caught her. Of course, her father had to turn the situation on Tom Robinson. Bob Ewell was trying to save Mayella's reputation. Mayella did an unspeakable offense in the 1930s. She was a white woman who tempted a Negro man. 

Mayella's father surely beat Mayella for her offense. Then Bob Ewell convinced Mayella to blame it on Tom Robinson. Atticus makes this clear in his defense:

We do know in part what Mr. Ewell did: he did what any God-fearing, persevering, respectable white man would do under the circumstances -he swore out a warrant, no doubt signing it with his left hand, and Tom Robinson now sits before you, having taken the oath with the only good hand he possesses - his right hand.

No doubt, Atticus proved that Tom Robinson could not have beaten Mayella. Mayella had been beaten by a left-handed attacker. Tom Robinson's left arm was crippled. Tom could not have beaten Mayella. Truly, Mayella is guilty of trying to convict an innocent man. She lied in court to save her own reputation. She protected her father who actually did the beating.