Just for fun, I would say it is not wrong to put nail polish on.
The wow of seeing one's fingers and toes sparkle is kind of fun. It is when selfworth gets caught up in it that there is a problem. If you make a big fuss over how pretty she suddenly is, well, that sends a poisonous message that pure, precious her isn't. I knew a lady who felt so little of herself she could not go outside unless she looked "perfect." She obsessed for hours over her nails, make up, hair, clothing, and figure. The poor thing ran outside with everything but secure self worth and a real smile! The best self worth is when we know pure, miraculous us is lovable, is LOVELY. That same lady has since thrown out all her trappings and has returned to the loveliness a genuine self and a genuine smile brings.
I think when our daughters understand that the only thing make up "makes up" for is the inner light and smile that we are not shining, it looses its appeal as a substitute for genuine self worth.
I think if you teach her what really matters things will be fine.