I think that Lowry is trying to show us things that she thinks are wrong with our society in this book. I think she makes their society seem like it's different from ours, but she is really trying to encourage us to think about whether our society really is that different.
For example, we would say that we do not force our poor to live in terrible conditions in a place like the Fen. However, I think that Lowry is trying to get us to think about similarities with our own society. You could argue that this is really a similarity with our own society because we have islands of poverty within our rich country.
Another example of this would be the treatment of the sick and weak. Lowry is trying, I think, to get us to think about our own laws. She is implying that our society really does condemn the poor and weak because we do not really take very good care of them through our social welfare programs.
So I think that Lowry is trying to get us to think about how we really are like this society.