On the whole, Mayella Ewell comes across as a rather pathetic, pitiable figure. She is poor, lives in a crowded dump with her many siblings and violent and abusive father, and she has no friends. Scout comments on all of this in a stark moment of realisation during the trial:
...it came to me that Mayella Ewell must have been the loneliest person in the world. She was even lonelier than Boo Radley, who had not been out of the house in twenty-five years. When Atticus asked had she any friends, she seemed not to know what he meant, then she thought he was making fun of her. She was as sad, I thought, as what Jem called a mixed child: white people wouldn’t have anything to do with her because she lived among pigs; Negroes wouldn’t have anything to do with her because she was white (chapter 19)
Here Scout pinpoints the cause of Mayella's loneliness: because she is poor white trash, other white people want nothing to do with her, and because she is white, the blacks also ignore her. Scout further realises that Tom Robinson was probably the only person who ever reached out kindly to her, but even all this went wrong when her brutal father intervened. In short, Mayella has no-one to rely on.
Lee also encourages our sympathy for Mayella with some little details that show she has tried to make the best of things, most notably with the beautiful geraniums that she keeps in her window, symbolic of her search for some beauty and meaning in her harsh, sordid, lonely life. However, although the reader can certainly feel sorry for her, her actions do help to convict an innocent man, and she indulges in a show of petulant behaviour on the witness stand so that our sympathy for her is ultimately lessened.