It's said that they are people who have no internal monologue and can't imagine pictures/images in their mind (they have aphantasia).
The reason I ask though is because I have never met a single person who doesn't have an internal monologue and who can't imagine images in their head. I don't believe such a person actually exists.
There are some "psychological surveys" which seem to indicate that some large percentage of people have no internal monologue. But, psychology, along with sociology, is pseudo science trash. I see no reason to believe that something like ~40% (or whatever huge amount it is) of people don't have an internal monologue, when I have not met a SINGLE person in my life who lacks one (and I've met hundreds of people in my life).
I doubt anyone reading this has met a single person who doesn't have an internal monologue and who can't visualize things in their mind.
If an NPC is just a person that doesn't think much about existence or life or deep philosophical questions, such a person is simply "stupid", not an "NPC". Unless the terms are synonymous in which case, just call such a person stupid.