In Season 2, Episode 2 of Big Mouth ("What Is It About Boobs?"), an evil version of Missy comes out of the mirror:
Is this a reference to the movie Us (2019)? Later in this Big Mouth season there is a more explicit reference to the movie Get Out. Both Us and Get Out were written and directed by Jordan Peele, who voices a few characters in Big Mouth.
The only thing keeping me from being sure that evil Missy is a clear reference to Us is that this season of Big Mouth was released in October 2018, while Us would only be released in March 2019. Since both Us and season 2 of Big Mouth were (likely?) in production during most of 2018, could this is an easter egg Peele managed to slip in?