Through Miller's meandering narrative we learn to understand one main thing: him.
All of what we learn and know by the end of Tropic of Cancer comes directly from his mind—complete with attitudes, opinions, perversions, and rubbish. Miller idolizes Walt Whitman and sort of sees himself as a 20th-century version of the humanist poet and essayist, but he also doesn't shy away from criticizing Whitman's idealism, which in all fairness was set in the previous century. As Miller announces:
When I think of this city where I was born and raised, this Manhattan that Whitman sang of, a blind, white rage licks my guts. New York! The white prisons, the sidewalks swarming with maggots, the breadlines, the opium joints that are built like palaces, the kikes that are there, the lepers, the thugs, and above all, the ennui, the monotony of faces, streets, legs, houses, skyscrapers, meals, posters, jobs, crimes, loves. […] A whole city erected over a hollow pit of nothingness. Meaningless. Absolutely meaningless. (5.30)
So do we have another Walt Whitman on our hands here? Or is Miller a totally different breed?
Remember that Henry Miller is a faceless narrator. We might know a little about the real, historical Henry Miller, but our narrator is all words. We get to see what everyone else looks like in Tropic of Cancer (basically just variations on ugly), but we only know Miller through his narration.
It doesn't take long to realize that he has a (bad) opinion about almost everyone, so we can't exactly trust him. We mean, Henry was probably no peach either, but since he's in control of the narrative, we have to sort of go with it. That's what we at Shmoop call an unreliable narrator.