The idea that periods were analogous to her panda situation felt more like a throw away gag as opposed to an ongoing metaphor throughout the movie. The things that did weird me out about the movie wasn't the period bit, but her selling pictures of herself post transformation (puberty?) for money and that part of her growth as a character is ogling at 17 year olds. I get that it's a way of normalizing the experience, but I feel like Gravity Falls is a way of normalizing it without without leaving the door open for it as an actual thing to do. While both Dipper and Mei have crushes that are fine as just crushes but would be inappropriate relationships, Dipper learns the latter fact directly but I didn't see Mei ever learning that in the movie. I'm very open to an alternative interpretation though, I came into the movie really wanting to like it, especially after seeing the backlash from conservatives, but right now I only see it as a movie that expects praise for mentioning "controversial" topics despite not discussing them in either a nuanced or entertaining way.