Atticus could be afraid that the jury will believe that her lies are true just because she is white. It is obvious to the reader that she is lying but the narrator has to use language like her performance or recital to let us know that she has practiced what she has to say. If you are telling the truth, you don't have to practice it, you just have to remember it. The jury doesn't have either the benefit of living 80 years after the incident like we do, nor do they have the benefit of having the narration of Harper Lee. All they have are Atticus' leading questions and Mayella's white testimony. Atticus fears the jury assuming lies to be truth.