Another possibility is that it could be a biographical genre. If the story is mainly about a particular mother's life and her experiences, and it's told in a factual - like way while still having elements of storytelling, it could be considered biographical. However, without knowing more about the specific 'Mama based on a true story', it's hard to be certain.
It might be a thriller or horror. Sometimes, real - life events can be quite dark and spooky, and if 'Mama' has some elements of mystery or danger related to a mother figure, it could fall into this genre. For example, if the true story involved some sort of unexplained events associated with a mother in a haunted place or something like that.