At Tom Robinson's trial, Mayella reveals herself to be much like her father in several ways, but one significant difference between them emerges. Like her father's, Mayella's behavior is hateful and antagonistic. Her testimony is frequently bitter, sarcastic, and belligerent. Like her father--and most of Maycomb--she is a racist.
Mayella's character, however, is significantly different from her father's in that she is frightened and vulnerable. When she grows a little more confident in her testimony, hers is described as a "stealthy" kind of confidence, not "brash" like her father's. In order to survive, Mayella has learned to view the world with wariness. When Atticus treats her with respect, she assumes he is mocking her. She is not accustomed to kindness.
There is a softness in Mayella that Bob Ewell lacks. She longed for human companionship and love, which led her to attempt the seduction of Tom Robinson. Scout realizes that Mayella "must have been the loneliest person in the world." In Scout's estimation, even Boo Radley's loneliness was less intense than Mayella Ewell's.
There are many similarities between the two; they have the same background, a lack of education, the same defensiveness when Atticus questions them. However, Atticus refers to some things that set Mayella apart. He hints that she was very lonely, so lonely that she turned to Tom Robinson for companionship. Given Bob's rather insensitive comments about negroes in chapter 17 ("lived down in that nigger-nest...they're dangerous to live around 'sides devaluin' my property"), this is a big difference between the two of them. Mayella rose above any preconceived notions about race in order to be with Tom.
Another difference is that Mayella is the one that cares for the children and runs the household, rather than her father. She describes survival, clothing and feeding the children, and how "Papa drank...sometimes went off in the swamp for days". So she is the more responsible one of the two. There is also the image of her flowers; earlier in the book the Ewell place is described as total squallor and chaos, but Mayella tended some very pretty red flowers, and they stood out amongst the tumult. This indicates that she tries to install order, seeks after beauty, and is trying to improve her station. Her father, who "was the only man I ever heard of who was fired from the WPA for laziness", is certainly not trying to better his situation.