The answer to this excellent question can be found at the end of Chapter Seventeen, which is when Mr. Gilmur asks him if he is ambidextrous. Note how Bob Ewell responds:
I most postively am not, I can use one hand good as the other. One hand good as the other...
Bob Ewell's ignorance of the meaning of the word ambidextrous is not important. What is important is that it has just been displayed that Bob Ewell could have been responsible for beating up Mayella, as she was beaten up mostly on the right side of her face and body, which would point towards a left-handed person committing this crime. Even though Scout is not too hopeful about the weight of this piece of evidence, it is certain that Jem believes this is a crucial piece of the puzzle surrounding the mystery of what happened.
After Mr. Gilmer asked Bob if he was ambidextrous, he replied "I most positively am not, I can use one hand good as the other. One hand good as the other." Chapter 17 takes place in the courtroom. Bob was being cross-examined by Mr. Gilmer when the question was asked. Mr. Gilmer was trying to determine if Bob was responsible for beating Mayella and blacking her right eye. The prosecutors assumed a left-handed person was responsible for beating Mayella and Bob’s response to the question determined his innocence or guilt or at the very least, his ability to commit the crime.
In chapter 17