Okay, so, if you're used to reading Modernism à la Virginia Woolf or James Joyce, you might be scratching your head about this genre choice for Nostromo. Bear with us, though, as we make our case…
Yes, there are lots of ways in which the novel is pretty traditional. Conrad's sentence structure and writing style are relatively clear (and complete, thanks Conrad!)—so, in that respect, it's definitely a far cry from, say, Ulysses. Also, Nostromo gives you healthy doses of action, intrigue, and strange/remarkable events, things that are often not really at the forefront of Woolfian and Joycean Modernism, which tend to focus on everyday events.
Digging a bit deeper, though, we see that that Conrad definitely gets experimental when it comes to narrative structure and plotting. You might have noticed, for example, that the book puts its supposed protagonist on the backburner for a good chunk of the novel. You could argue that Nostromo doesn't really become a major focal point until roughly halfway through, which is when the narrative really starts dipping into his mind and clueing us into his motivations.
By making us wait to "meet" Nostromo and giving us only other people's perspectives on him for the first part of the novel, Conrad's narrative mimics the way the characters kind of treat him as a mere tool in their own stories rather than a whole person. So, like other Modernists, Conrad definitely uses experimentations with the novel form to a larger thematic effect.
Then there's the way Conrad rapidly shifts focus back and forth between the minds of different characters, which is definitely Modernist—hello, Mrs. Dalloway. You get a great example of this technique when Nostromo and Martin are on the lighter, trying to avoid being seen by Sotillo and his crew on the steamer. The narrator kind of "floats" back and forth between the two groups, giving us their differing P.O.V. during that tense period. Check out one snippet from that section and see for yourself:
Sotillo, on the bridge, muttered from time to time angrily to the captain. […]
"If your eyes are of no more use to you than this, I shall have them put out," he yelled.
The captain of the steamer made no answer, for just then the mass of the Great Isabel loomed up darkly after a passing shower… […] The ship was put then full speed on the course, and a great bustle of preparation for landing arose among the soldiers on her deck.
It was heard distinctly by Decoud and Nostromo. The Capataz understood its meaning. They had made out the Isabels, and were going on now in a straight line for Sulaco. (II.8.50-54)
In this moment, the narrative shifts focus abruptly from Sotillo and his crew's thoughts to Nostromo's. That kind of thing happens a lot in that chapter and throughout the book, and it's definitely the kind of trick you see a lot in Modernism.