The theme of The Misfits is rooted in the idea of tolerance and acceptance of others.  It seeks to embrace the idea of what should be as opposed to what is.  I think that this is important to the novel because Howe sought to write a work that spoke out in favor of social acceptance and how the ability to display solidarity can transcend most other fundamental challenges that are faced by adolescents.  The theme is important because its message is deliberate on the part of the author:

In The Misfits and Totally Joe, an underlying theme that kept surfacing had to do with my own feelings of being different as a boy and then a man. My own shame about being gay, my own discomfort, my own wish that I could be open and accepting and be accepted. These feelings kept bubbling up in my work, which often celebrated difference and feeling good about who you are.

The theme is important to the work because it seeks to explore a personal dimension with social implication.  In this light, the novel's theme is important to understand the literary meaning and social relevancy of the text.