In Harper Lee's To Kill a Mockingbird, I believe the author does agree with the statement, "The oppressed often display a greater cohesion than the more well-to-do of a community."
Lee is saying that those within the society that struggle to survive, stick together more so than those who don't struggle, but live comfortable lives. As an example, I believe Lee would be referring to the black community more so than the poor white community. The poor white community has little to offer one another financially, but they also don't seem to be spiritual joined, and in the novel, Bob Ewell wouldn't lift a hand to help anyone.
However, the black community has a strength in the way in which its members support each other. We see this when Tom Robinson goes to court. The balcony is full of black men and women there to support Tom. When Scout and Jem go to church with Calpurnia, the pastor passes the offering plate to take a special collection for Helen Robinson because with Tom in jail, the family is struggling to get by.
When Atticus leaves the courtroom at the end of the trial, the blacks all rise, and Rev. Sykes advises Jem and Scout to do the same out of respect for Atticus. The black community has little enough to survive on, but when Atticus is done defending Tom, the members of that community leave gifts of food for the Finches out of respect and thanks for what Atticus has done, and their generosity brings him to tears. These are people who have been forced in some ways, and have chosen in other ways, to stand together in support of each other. They may be oppressed in terms of their race and their poverty, but spiritually, they are strong when united, stronger than any of the wealthier members of the society. They are united because they have to be.