the flatness of the characters had a lot to do with the fact that in the "bell jar", she was blind to other people. depression and self-involvement are often closely related. Also, she was in her early 20's, the height of looking inward. I believe she painted the characters in a dull way because that's how she saw them, whether they were chasing the American dream, trying to dominate her, etc. To a depressed person, color and variation tend to fade into the background.