I take the 'pick your battles' view. If something is temporary (eg false eyelashes or lipstick) it can do no harm, and lets teenagers express themselves. However awful we might think it looks, and bearing in mind that they are not remotely interested in impressing our generation, they will grow out of it. Making a big deal out of something small is more likely to result in a major rebellion down the line, IMO.
What may seem like small things can be so important at that age, and if the girl is getting upset, why does it matter if she wears them? At about 13/14 I was desperate to pluck my eyebrows (in line with the fashion of the time) and my mother was adamant that I was not allowed. I had quite thick brows, which would be fashionable these days, but I hated them, and looked different from my friends, who all plucked theirs. I remember being really upset and self-conscious, and for what? To allow my mother to feel 'in control'?