Mayella Ewell is not symbolized by the mockingbird referred to in the title of the book, To Kill a Mockingbird.
According to the passage from which the title originates, "It's a sin to kill a mockingbird" because "mockingbirds don’t do one thing but make music for us to enjoy." In other words, you can't kill mockingbirds because they don't do anything to hurt anyone.
Mayella Ewell is hurting Tom Robinson with her accusation of rape. Therefore, she cannot be the mockingbird from the title. However, there are at least two other characters who do fit that description: Tom Robinson and Boo Radley. Tom, on trial for rape, has never hurt anyone as far as we know, yet he is being persecuted by the Ewells as they try to cover up their own shame. Boo Radley may have been a wild young man years before, but he no longer deserves to be bothered and slandered by the neighborhood children, who make him out to be some kind of subhuman monster until he saves their lives at the end of the story.
She, too, is a kind of mockingbird, injured beyond repair by the forces of ugliness, poverty, and hatred that surround her. she is actually a good person that likes to keep clean and doesnt like acting like her father but she is abused and possibly molested by her father, while she has to deal with her unhelpful siblings. She has absolutely no friends and has been treated cruelly all her life.