Cold and Disinterested, with a dash of Mirth

Since the novel spends a lot of time "in" the characters' minds, the tone is often dependent on who is speaking. However, there are definitely some moments when a "voice" exterior to the characters emerges. When Hirsch is being tortured, for example, we get some commentary on his predicament that isn't particularly sympathetic (so it's fairly clear it's not coming from Hirsch himself):

Sotillo, followed by the soldiers, had left the room. […] Hirsch went on screaming all alone behind the half-closed jalousies while the sunshine, reflected from the water of the harbour, made an ever-running ripple of light high up on the wall. He screamed with uplifted eyebrows and a wide-open mouth—incredibly wide, black, enormous, full of teeth—comical. (III.9.24)

As you can see, the presentation of Hirsch's pain is fairly causal and disinterested—check out the pretty water reflections on the wall—and the narrator even goes so far as to drop the word "comical" in describing Hirsch's face as it contorts in agony. Being tortured lol. The moment is consistent with the general "let-me-chuckle-to-myself-as-you-idiots-kill-each-other/yourselves" vibe that we get here and there from the narrator.

No doubt about it, Conrad was a cynical dude. And why shouldn't he be cynical? When you spend the your life up to the age of thirty-seven running around the world and then sit down to write some of the century's best novels (in your second language, no less) you kinda get to be cynical.