Here is my estimate:
20% genuinely are reaping what they sow. These are the extremely lazy and the extremely stupid and they have solely themselves to blame.
60% have a lot working against them, but they aren't really doing much to help themselves either. These people aren't entirely to blame and they may have faced significant issues with their upbringing, racism, etc. Despite having it hard, they probably could break out into the middle class if they really worked at it, but it is a lot easier to fall into a pattern of blaming external factors than it is to go to night school. To be honest, if I had been born into a very poor family without much guidance, I could see myself falling into this category.
20% get genuinely shafted. This is the single mom who works 2 jobs and goes to night school to become a nurse, but is forced to drop out when a relative becomes ill and needs care. Good person, hard worker, kept down by really crummy circumstances.