I think it comes from the fact that if you have a vagina and the accompanying microscopic perineum then you should not wipe back to front, as that could potentially move bacteria from fecal matter towards the vulva, where it could get into the urethra and cause a UTI or any number of places and cause all sorts of undesirable issues, and thus doing it is “stupid” as in a bad idea. From that it became a more general way of saying someone is “stupid” like “you sound like you ate play-doh/glue/whatevs as a kid”. Got picked up on some twitter-adjacent parts of the internet and now it’s slang.