I think because you are an adult and her mum rather than her bezzy mate so you have to stand for what’s in her long term interest and welfare rather than mirroring or enabling poor decisions she makes, otherwise you are blurring lines and helping her rationalise poor decisions.
Yes, she will probably get it done the day after she turns 18, but maybe not. Even if she does, she knows it’s something her trusted mum doesn’t think is necessary or a good idea and that seed may help her stop escalation or lead her to stop earlier than she might.
DD1 has talked about getting a tattoo a few times over the years. I would never have paid for it or given my consent while my consent was required, I just say that while I know they are super common and I have friends with them, I always think about if I’d been forced to wear my first (Deidre Barlow/ Dennis Potter) specs for the rest of my life, and that’s what you do when you get a tattoo. She’s going travelling soon and talked in the past about getting one done to commemorate the experience. I get that, and while I’d prefer she didn’t of course if she does, as she’s an adult, I won’t be foaming at the mouth or nasty about it. It’s up to her.
if you have a good relationship with your daughter she will know it’s coming from a good place, even it she thinks you are wrong, and I think it’s reassuring, even for adult children, to know there is someone who has their back even when it doesn’t make them Mrs Popular.
I’m not saying it’s the end of the world if she gets lip fillers, - to be clear. It’s not the end of the world if my DD1 comes back from travelling with a tattoo.