I'm serious about this: I worry about what this means about your relationship, as it seems to be literally more than skin deep. When it comes to choices about appearance, not much could be more personal than one's face. It's not as if she's asking you, "Do you prefer my hair long or short?" Or, "Is this color becoming?" You have every right to offer your opinion when it's solicited or when the relationship is intimate enough that you know you can say something like "that style just doesn't bring out your best, hon." But when it's her face and she wants to tweak its appearance, it would be great if you could say something like, "That kind of decision is completely up to you, and I'll support whatever you choose. But I want to be sure you know that I don't think you need it, I love you exactly the way you are, and I hope you won't expect me to follow suit because it's just not my thing." The way you're talking about it makes it seem more a political stance than a personal one, and you're a bit too righteously indignant about your lack of vanity. My guess is that you probably can't tell when she has had it and when she hasn't. Try to be more generous in spirit, okay? Just because some people turn themselves into Barbie dolls doesn't mean that's what's happening here.
Best of luck.