Was Erin a Mary Sue
It sure seems so, she solves everything and is able to single handedly destroy the villains without setbacks.
shareA Mary Sue is an artist's self-insertion into a work. I don't really think Adam Wingard or Simon Barrett dreamed of putting themselves in Erin's position. They were merely just subverting artist's usual expectations of the final girl, in that she just happened to be more capable at survival than the assailants were at committing murder.
Erin didn't seem to have the respect or admiration that a Mary Sue usually gets, either. I don't think she was particularly well liked amongst the family. Granted, they didn't even like each other either.
I think Erin had setbacks. She hurt her leg really bad jumping out of the window, she made questionable moves like throwing the pot of cooled oil at an attacker, and had a lot of luck. Hell, she might be arrested for murder after the credits rolled ("suspect?"). I think she just had the right amount of luck and skill.