I liked this movie a lot, though I’m not sure if I have the intended reading of it. It seemed to me like a movie whose horror was more about mental health than anything else, the fear of someone doing irrational, harmful things and not being able to do much aside from getting them involuntarily committed (which, absent thinking wishes are real, is absolutely what he should have done). There’s a sense of helplessness that pervades the film, and some of that is down to our trashbag white guy with no dreams or ambitions, but I think it makes for effective horror if that horror is of the “my girlfriend is acting absolutely nuts” flavor. They do mention at various times that she appears to be going through some kind of psychotic episode.
As a monkey’s paw curling kind of thing … it doesn’t really work for me? The thing he wishes for is that she would love him, and what comes out of the wish is not even remotely that; the classic monkey’s paw compliance would be that she loved him too much, but instead she gets replaced by a demon or whatever, and there’s just not that much connection between his obsession with her, and her obsession with him. She’s a girlfriend with borderline personality disorder, or the heightened horror version of that.
(There is also the horror of Nikki’s lost autonomy, but this really seemed like it was secondary to what the film was trying to do, at least to me.)