Alright, I'll bite. As someone who's an avid Nier fan and was playing Reincarnation well before this event was unveiled, the narrator of Mama does have a tendency to describe things as the character in focus sees them. As I see it, she's relaying information as if she's telling a story to the protagonist. However, I would agree with the idea that certainly not all of what is said in the text is reflective of how Emet would describe it. I'm afraid that landing on a square answer of what is objective and what is not is pretty much impossible, as the narrator is a very clear 3rd party that is equal parts describing surroundings as she is reading into the character's feelings.