What would be a good thesis statement for this book?

There are two ways that I recommend going with a thesis for this book. I would either have a thesis that focuses on a character analysis of Helga, or I would have a thesis that does an examination of one or two of the major themes present in the book. Make sure that the thesis statement is making some kind of argument that the paper is required to prove to readers. The following might work well: Although Helga is motivated to find her place in the world, her spirit is eventually beat down by the quicksand of the racism and sexism that exists in the world around her.
This thesis is specific in its intent; however, it is broad enough to give your paper the freedom to analyze two themes, as well as provide a character analysis of Helga. If those themes don't resonate well with you, then use the same thesis template and swap in whatever theme(s) you feel strongly about.