Tom Robinson could not have beaten up Mayella Ewell because he is crippled on his left side.
Early in the trial, Atticus establishes that Mayella was attacked by a left handed person, and that Bob Ewell is indeed left handed.
Atticus was trying to show… that Mr. Ewell could have beaten up Mayella. …If her right eye was blacked and she was beaten mostly on the right side of the face, it would tend to show that a left-handed person did it. (ch 17)
Scout is not impressed. She figures that Tom Robinson could just as easily be left handed, and then there is no reason for all of the fuss. After all, she is a child. She can follow most of the proceedings, but she does not see the bigger picture.
We see that Tom Robinson is crippled when they try to swear him in and he can’t keep his left hand on the Bible.
Thomas Robinson reached around, ran his fingers under his left arm and lifted it. He guided his arm to the Bible and his rubber-like left hand sought contact with the black binding. As he raised his right hand, the useless one slipped off the Bible and hit the clerk's table. (ch 19)
Tom could not possibly have attacked Mayella Ewell. Since Atticus went to so much trouble to demonstrate that she was attacked by a left handed person, it did not take much to get the jury’s attention when they saw that Tom could not use his left hand.
Unfortunately, being physically unable to commit the crime was not enough for the jury. It was still a black man’s word against a white woman’s.