You're probably not surprised to hear that Tropic of Cancer doesn't fit snugly into one genre category. Miller is all over the map, just as he intended.
We've definitely got some autobiography going on, as it is the story—however embellished—of Henry Miller's life in Paris. But we also know that our author adds and subtracts at will. So fictionalized autobiography might be more accurate.
For example, Miller's long-time affair with Anaïs Nin goes completely unmentioned. Sure, some critics have identified Tania as a stand-in for Nin, since almost all of the characters in the book are roughly based on real-life people from his time in Paris. But because we know Nin was a major part of his life—she gave him a typewriter and funded him with cash from her rich husband, Otto (a source of income that goes conspicuously without mention here)—he is probably leaving a lot of other stuff out, too.
Bottom line: we have some serious myth-making going on in here.
We threw in the dystopian and philosophical tags because Miller has so many stinkin' visions of destruction and death. This is a man reduced to begging, living hand to mouth, mooching off friends, and working minimum-wage jobs—but he still celebrates life, mocking and scorning everyone and everything. He hates institutions and anything conventional, and rejects manners and propriety.
Most of all, he looks into his crystal ball and predicts that man, nation, and civilization itself will lead nowhere and may even end in apocalypse: "The world is a cancer eating itself away […] it is the world dying, shedding the skin of time" (1.8). Why? According to our guy, because men have no purpose, meaning, or passion.