Why was it a mistake for Tom Robinson to say he feels sorry for Mayella? How will it affect the jury?

Basically, for Tom to say that he felt sorry for Mayella was to violate the unwritten racial etiquette of Maycomb and of the Jim Crow South more broadly. He was rising above his station by saying that he felt sorry for her. Though Mayella lived in grinding poverty and had a seemingly useless father, she was white, and that, in the eyes of many people, made her superior to Tom. Scout immediately perceives that Tom has erred in making this statement, and Gilmer, the prosecutor, immediately seizes upon it by sneering at the very idea that the accused man could feel pity for a white woman. Atticus tries to address it in his closing statement when he describes Tom as a "quiet, humble, respectable Negro who had the unmitigated temerity [nerve, or audacity] to 'feel sorry' for a white woman" (204). Atticus is mocking Gilmer's emphasis on this testimony from Tom, but Scout correctly recognizes that it will not help Tom's cause, and that it risked alienating the all-white jurors.