A simple but really cool premise that you implemented very well here. I liked that you could choose how the game became harder, but I feel like more monsters quickly became the best option since eventually you just have all monsters.
The music is good, but it's a bit jarring to transition from day to night.
I do feel like it needs to be made a bit more clear what was and wasn't a mimic, since the giant red x covers up most of the sprite, and the selection screen covers up a good portion of the screen as well, it's hard to tell.