Pilled is a suffix from "red-pilled" (a Matrix reference) meaning converted to a belief or aesthetic. "Cottagecore-pilled" = into the cottagecore aesthetic. Originally adopted by far-right groups, now used broadly and ironically. Not concerning unless your teen is using overtly political pills ("black-pilled," "based-pilled"); worth a chat then. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in.