In this novel, a mockingbird comes to symbolize a person who brings beauty into the world and never inflicts harm on anyone. In some ways, this description could apply to Mayella Ewell. After all, the young girl does try to bring beauty to her little bleak, rundown home with the limited resources that she has available:
Against the fence, in a line, were six chipped enamel slop jars holding brilliant red geraniums, cared for as tenderly as if they belonged to Miss Maudie Atkinson.
Mayella longs for a more beautiful life, and her flowers show her hope for a life apart from her poverty-stricken existence and her abusive father.
Strictly speaking, however, Mayella isn't a mockingbird, and this is because she doesn't only make the world more beautiful—she also acts in ways that are selfish and harmful to others. She lures Tom Robinson to her house so that she can make romantic advances on him. When she's testifying at the trial, she screams at Atticus, who shows her nothing but compassion. Most damning of all, she refuses to tell the truth about her encounter with Tom Robinson; her false testimony sends Tom to jail and ultimately to his death.
You could certainly argue that Mayella is a character who deserves pity. She is left to care for all of her younger siblings while her abusive father drinks up the money they need for food. She herself has no one to bring a touch of beauty into her life, which is why she is drawn to the unfailingly polite and kind Tom Robinson in the first place. She has only ever known hate and abuse, and she has experienced so little goodness that she cannot understand Atticus's kindness to her when she is on the stand.
Still, if we look at the symbolism of a mockingbird in the book, it's clear that Mayella is not a mockingbird in the same way that Tom and Boo Radley are. For this reason, it could be argued that Mayella is a more complex character than Tom and Boo, as it is difficult to characterize her as wholly good or evil.