Why is the Ewell family important in the novel To Kill A Mockingbird?
Maycomb is steeped in racial and social class distinctions. Many of the white citizens view the black citizens as second class. Many of the wealthier citizens view the poor as second class. This is why Aunt Alexandra doesn't want Scout to be friends with Walter Cunningham Jr. The Ewells represent the lowest section of this social strata and this is a result of those traditions but it is also the product of Bob Ewell's awful treatment of his children and his generally selfish ways. If Bob Ewell had become a productive member of society and treated his children with some decency, the Ewells would not be as hated (although if they remained poor, the wealthy would still tend to look down upon them).
Atticus is one of a few who actually pities the Ewell children. Bob and Mayella both represent the disgruntled, downtrodden, ignorant portion of society. Bob is the antagonist of the novel and it's hard not to say he is simply the embodiment of evil.
Mayella is also an antagonist with respect to Tom Robinson but in the grand scheme of things, she is also a victim because she's been the subject of an abusive father and has been denied an education. In fact, Tom stated in the trial that he felt sorry for her as well. "I felt right sorry for her, she seemed to try more'n the rest of 'em." (Chapter 19)
While the Ewells are looked down upon because of their economic status, their isolation from the rest of the town is self-imposed - by Bob. Therefore, it is more accurate to say that Bob is the antagonist and his children are his unfortunate, unwitting pawns.