Scummy mummy is informal British term for a mother who openly admits to imperfect parenting (e.g., let kids watch too much TV). Often self-applied with humour. Not concerning, mostly affectionate. 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. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.