Sometimes I will see a good looking girl with a noticeable mustache. Not like a full on mustache but you can tell it's there. There are times when I see these kind of girls and I want to tell them "Hey, no offense but you really got to shave your mustache." But I never have the balls to say that. Not trying to be rude at all; I just genuinely want to help her look better. Do you think its rude when someone tells a girl to shave her mustache? (Or I may be completely naive and not know about girls who like to grow their mustaches)

Notice I said "most", "a lot", and "some." Not "all."
shaving is not the answer for girls-it's electrolysis or laser hair removal-they carry some risk but is a permanent solution for unwanted hair.
Maybe they keep it to keep away men.
One thing's for sure, if you say that in a social environment she will HATE you for the rest of her life and never forive or forget it.
You would lose out more than she would.
Or they just like mustaches.
Second, females should not shave a 'moustache', because it will grow back thicker. They should have them waxed or removed.
If you find this female attractive, tell her that. Then suggest to her that she would be more appealing to you if her upper lip hair were not as prominent. Let her know that you would love to go on a date with her IF she waxes her lip.
Careful not to use this technique to coerce people into doing things that could be permanently damaging and/or irreversible, such as breast implants, penis enlargement, etc.
Q: What's the difference between a blonde and a walrus?
A: One has whiskers and fishy flaps, while the other is... well, a walrus.
Maybe that's the look they're after?
Tell her and let her TAKE IT LIKE A MAN, what?
Maybe hint to one of her chick friends so they can tell her?
Secondly, it is her choice. If she wants to have a moustache she can go ahead and have. If she wants to get rid of it; again, it is her choice.
And I think that you would be just another ignorant person in this world if you ever do what you intend to do. Whether she is hot or not, it is none of your business how she lives her life. If it does not matter to her why would it matter to somebody whom she does not even know?
It is obvious from your post that you have a very shallow view that would affect any girl if you were to, out of nowhere tell her to shave her moustache. It does not surprise me that there are so many guys like you in the world as it seems that so many have been brainwashed by the use of photo shopped women in magazines and on television.
You would think that somebody would have one bit of decency in letting somebody be the way that they are. It is not her fault that she has a moustache. Why would she look any less beautiful than another girl? Why is beauty criticised in such a manner when I am sure that not you or anyone would be comfortable with having to live with something that is viewed as ugly in this world?