Sadly for Tom Robinson, Mayella's weaknesses far outweigh her strengths. As Atticus pointed out during his summation to the jury,
"... she tempted a Negro. She did something that in our society is unspeakable: she kissed a black man... No code mattered to her before she broke it, but it came crashing down on her afterwards." (Chapter 20)
Mayella's overwhelming desire for the attentions of an adult man led her to try and seduce Tom, and her lies afterward led to his conviction and eventual death. After she was seen kissing and hugging Tom by her father, she took a beating from Bob; her fear of him led her to go along with the story that Bob probably concocted, that Tom had initiated the contact and had later raped Mayella. Mayella was not strong enough to tell the truth, so she went along with Bob's story, knowing that the all-white jury would accept the Ewells' version. Though Mayella "is a victim of cruel poverty and ignorance," her own lack of moral backbone leads her to allowing Tom to accept the blame for her own actions, leaving the man who she so desired to be wrongfully convicted.
Atticus alludes that it is probably not the first time that Bob has beaten his daughter, yet Mayella remains in the Ewell house, where she is expected to take care of her younger siblings. This is one of her few strengths, the willingness to stay at home and give up any chance of a normal social life by taking the place of her mother in raising the kids. She was "a thick-bodied girl accustomed to strenuous labor," and she successfully pinches her pennies for a year, saving "seb'm nickels" to send the children to town for ice cream. She also manages to bring a touch of beauty to the squalid conditions of the Radley property with the red geraniums she so carefully nurtures. And she shows a stubborn streak at the trial, refusing to admit the truth under Atticus's strenuous cross-examination as he "rained questions on her." She stood her ground, calling the men in the courtroom "yellow, stinkin' cowards," and
She was as good as her word. She answered no more questions, even when Mr. Gilmer tried to get her back on track. (Chapter 18)