yeah, I agree with the two other guys. girls usually wear makeup because they want to look prettier, so just assure them that you think she's beautiful and just gently say that you'd prefer if she didn't wear makeup.
but then again, you might have to see...i personally wear makeup because I want to hide my hideous acne scars, and if I don't wear any makeup at all, I refuse to leave the house. so if you think she might be in my situation, just tell her to lighten up on the makeup, because telling her to wear none might be a touchy topic.
You need to accept that she enjoys make up...Sure you can tell her she's beautiful without it but you have to let her be herself...Period...The dislike of make up is your issue not hers...So it's more something that you have to deal with...
People wear make up for different reasons, you have to accept her for who she is, make up and all, if you try to be controlling over this you might turn her off you... also if you succeed, who knows how controlling you might end up in this relationship... beware and relax, she may ease up on it naturally... Good luck :-)
jus keep tellin her how naturally beautiful she is an that she doesn't need make-up she prob wears it to make herself feel better bout herself, so jus keep tellin her how great she is an doesn't need the stuff, that mite help