First-person narrators are rarely reliable, because the stories they tell are colored by their own blind spots, emotions, and agendas. Like most first-person narrators, Marlow is not entirely reliable.
First, the frame of the story points to it as a sailor's yarn, which is typically an exaggerated or dramatically heightened story. We are told, too, that Marlow had a "propensity to spin yarns." The context points to distortion: night has fallen on the deck of the ship, at rest in the mouth of the Thames. In the darkness, Marlow begins to tell a tale framed by an eerie, ghostly setting. Like all good sailor's yarns, we can expect his story not to be entirely accurate but to heighten the scary aspects of the drama for effect—this will be a story told artistically, not a straight recitation of dry facts.
Marlow uses elision, or the omission of facts or scenes, when he tells his tale. This can lead to distortion or a sense of mystery. Marlow, for example, states that "The horror! The horror!" are Kurtz's last words; but these are, in fact, the last words that Marlow heard. Kurtz dies off scene, while Marlow is eating dinner, so we don't know if Kurtz had more to say. However, the words "the horror" fits the point Marlow is attempting to make about the horrors of European imperialism.
At the end of the story, too, Marlow shows a capacity for saying what his listener wants to hear when he lies and tells Kurtz's fiancée that Kurtz's last words were her name. Ironically, those could have been his last words, but that is not what Marlow believes.
A reader should keep in mind that Marlow is playing to an audience. That doesn't mean he doesn't convey truth; but everything he says is not necessarily the literal truth.