Mama is a horror movie that is not a "splatter" movie, which means it does not rely on blood and pain and sadism for its effects, and that's all to its credit. How often do you see a horror movie rated PG-13?
But having said that, it still depends too much on things that jump at you out of the dark, and while there is good subtlety in the acting of Jessica Chastain and in the general conception of the character she plays, the all-important theme of children who do not realize that they are in touch with unholy things is hardly developed at all. The difference between the eight-year-old who may begin to suspect something wrong and the six-year-old who is too young to notice could have been developed to chilling effect, but perhaps at the cost of revealing too much about what is going on-- which I am trying to avoid also, and at considerable cost to clarity and completeness.
But with all my reservations about Mama, I don't want to discourage people who favor things like the various versions of Turn of the Screw from going to one of the very few movies aimed at their tastes.
Chastain's heroine is an interesting variation on the usual trapped lady trying to defend the kids. She isn't a whole lot interested in the kids and keeps protesting that they are her boyfriend's job, not hers. But she is certainly not paralyzed by fear or given to hysterical screaming, and she is a formidable adversary who doesn't wait to understand before she acts. The special effects are almost primitive, but I found them effective in their very simplicity.
All in all, Mama isn't a big deal. But in present context, I found it satisfying and a good deal more.