As Scout mentions early in the novel, "Atticus said the Ewells had been the disgrace of Maycomb for three generations." Sadly, Mayella had little to do with this assessment, but she was forced to live with the shame of her family name. Few people in Maycomb would have anything to do with the family, and the Ewells live in near isolation--between the city dump and the African-American neighborhood. Because of her mother's absence, Mayella was forced to look after the younger Ewell children, since Bob was usually out drinking up his government check. Mayella apparently had no friends, and her loneliness so overwhelmed her that she sought out the company of a married black man to comfort her. After her appearance on the witness stand, and after Tom's conviction, the townspeople probably scorned the family more than ever. No doubt Mayella received some sympathy from a few of the townspeople, but she probably had no more visitors than before. She is a victim of both society and her own family.
The location of the Ewell home is very telling: between the city dump and a segregated black community. This indicates that the Ewells live on the margins. They are not embraced by the mainstream white community due to their poverty, but they can distinguish themselves from the segregated black community due to their being white, a privilege that her father relishes. Their location next to the city dump emphasizes the town's condemnation of them as "white trash," a pejorative term for working-class or poor white people.
Mayella is a victim due to prejudices about people of her class. She is also limited by her gender. After her mother's death, she, as the eldest, was expected to take on the responsibilities of the household. The expectation that she remain at home to take care of her younger siblings isolates her even more, as it was not possible for her to go out to meet other people.
Finally, I would argue that Mayella is a victim of the limitations placed on white womanhood at this time. In the South, white women were forbidden to have interactions with black men outside of that of mistress and servant. If others in the community learned that there was any friendly or intimate interaction between a white woman and a black man, it could have resulted in the man being lynched.
Tom's life is at stake because Mayella has led others to believe that Tom attempted to rape her. Mayella uses the myth of the Southern woman as a flower of femininity to assert her place in the white community. She knows that most would frown upon a friendship between her and Tom and that knowledge of such a relationship could further isolate her family.
The myth of the Southern white woman as a flower of femininity is, to paraphrase Atticus Finch, a myth in polite society. Though Mayella does not know it, and the novel only suggests it, this myth is rooted in the idea of white women as the possessions of white men. This sense of possession leads to irrational hostility toward black men who are perceived not only as threats to political power but also as threats to the sexual and social dominance of white men.