With the idea that the story Fat City by Leonard Gardner clearly presents important universal themes, I would direct my thesis statement to those themes.
With this in mind, the story concentrates on two boxers who want to make their lives amount to something: one we could probably say is closer to the end of a boxing career, while the other is just getting started.
Our experienced boxer is Billy Tully, and our newcomer is Ernie Munger. Both men are presented with opportunities to pursue a cherished goal: to make themselves a name in boxing. However, in some ways, the men are very similar in that they don't have the focus or commitment to truly work hard for what they want. They allow their personal choices and the situations they confront to derail their forward movement. Tully's life falls into ruin, and unfortunately, by the end of the story, it seems Munger, even with so much potential and promise, is taking the same self-destructive path.
The themes I see here are:
...that life cannot come to you: you must meet life head-on. And it must be done with complete dedication and unwavering motivation. Everyone experiences obstacles, disappointments and heartaches, but many people are so committed, that nothing can stand in their way.
[Overall, the] message...from Gardner's book is that one must be truly dedicated to one's purpose in order to achieve it, and avoid those things or individuals that people sometimes invite into their lives that bring with them failure and loss of hope.
Keeping the above mentioned themes in mind, if I were writing a thesis statement for a paper, and had an understanding of the novel that would allow me to support my statement with specific examples from the story, it would probably read as follows:
In the novel Fat City by Leonard Gardner, the reader learns that life is full of challenges. Everyone must face a variety of obstacles on a daily basis, but only those with unwavering dedication can achieve their goals by remaining focused and committed to their purpose. To do so, it is vital to avoid situations and individuals that can slow or stop someone's progress in reaching his or her objective.
I hope this is of some help.