The Ewells and Cunninghams are both poor. That is the main extent of their similarities. Bob is clearly a racist and Walter Cunningham Sr. does express some of Maycomb's racist tendencies when he goes with the mob to get Tom Robinson at the town's jail. But he is thoughtful enough to persuade the mob to disperse at Scout's uncertain but effective interruption. Unlike Bob, Walter is a hard worker and he is honest. In return for Atticus' legal services, Walter could not pay, so he gave Atticus stovewood and hickory nuts, among other things, and this more than paid Atticus back.
The Ewells were a disgrace. And because Bob was such a lousy father, who took to drinking and abusing his children, the county allowed him to hunt and trap out of season. When Jem notes this seems illegal, Atticus tells him:
“It’s against the law, all right,” said my father, “and it’s certainly bad, but when a man spends his relief checks on green whiskey his children have a way of crying from hunger pains. I don’t know of any landowner around here who begrudges those children any game their father can hit.”
This is also the reason the Ewell children are allowed to skip school. The community doesn't expect Bob Ewell to change so they compromise and look away from what the Ewells do. The community does expect an amount of respectability from the Cunninghams because they've been relatively good citizens.
Aunt Alexandra focuses too much on social class based on family history and income levels. That's why she calls Walter Jr. "trash." She is, of course, being unfairly and unjustifiably ignorant. Thanks to Atticus's guidance, Scout realizes that Walter is nothing like the Ewells.