It depends on the perspective from where you're looking. From a reader's perspective, the Ewells are considered lowlifes because of the way they treat the blacks in Maycomb. They are so low on the social and socio-economic spectrum that they have to bully and persecute the only people who would have been considered "lower" than them in that society at the time- the Robinsons.
Scout initially sees Walter Cunningham as a lowlife because he is poor and when he comes to dinner with the Finches, she is apalled by his eating habits, but Calpurnia quickly puts her in her place and explains that she is no better than anybody else just because she has more luxuries than others.
From the perspective of those in the town who did not support what Atticus was doing, the Finches soon became "trash" to them. Atticus was "trash" because he sided with a black man. It was no longer relevant that the Ewells were the lowlifes of the town, because now it was a black man against a white man, and as a white man, Atticus should have sided with the other white man, regardless of what was right or wrong. There were very few people in the town who outwardly agreed with what Atticus was doing. By association, Jem and Scout became "trash" as well because they were the children of the man who was trying to help a black man beat a white man in court.
During that time, though slavery was no longer existent, there was still heavy racism, especially in the South. It didn't matter if the blacks were the most upstanding citizens; they were still considered "trash" to many of the white families then. Because of these attitudes, many people sided with Bob Ewell and against Atticus because they just didn't agree with what he was doing. Of course, from the perspective of the reader, those who are most prejudice- the Cunninghams, the Ewells, and Mrs. Dubose would be considered "trash," not because of their socio-economic status, but because of their strong prejudices and feeling of hatred toward the black people of the community.