Is the narrator of "The Fall of the House of Usher" actually examining the extent of their own nightmares or fantasies, or is Poe envisioning the downfall of humanity?

This is an interesting either/or question. Both interpretations can be defended as valid, though I would say both are atypical readings. What is interesting about the story is that it is so open to multiple interpretations.
A convention of storytelling is that we know Poe concocted the story: there is no house of Usher in real life and no Usher family. We are nevertheless meant to suspend disbelief and take the events recounted as "real." Therefore, the idea that Poe is writing a story that is, in itself, the fantasy of its main characters—a fantasy story within a fantasy story—is interesting.
Is the house of Usher a fantasy of the narrator: something that, even within the confines of the story's boundaries, does not exist? A case could be made that the events described—such as Madeline clawing her way out from the crypt and the house collapsing into the tarn—are so fantastic that they must be the narrator's dream vision. If so, the question becomes: what does that tell us about the narrator? Perhaps that he is a troubled soul running away from his own death wish?
Second, we could read Roderick and Madeline as symbolic of a humankind in decay and falling into ruin because of narcissism and self-obsession. Again, while unusual, a reading could be constructed that the tale serves as a warning to humans not to become too isolated from each other and the natural world.
I, however, tend to read the story as "real" within the context of being a fiction. I believe that the narrator "really" traveled to this place and met these people. I also take Roderick as an individual more than a symbol of fallen humanity, though that reading is more plausible to me than the first one. The important thing in interpreting a text is to back up with you are saying with evidence and quotes from the text.