I don't like my girlfriend's makeover and I tell her she looks bad. What do you think about that?
Why don't you like her makeover?
I don't like her makeover because it makes her look even older than how old she is.
While it's good that you are only making her look her best, it is still bad manners and taste to tell it to her face like that. She is not wearing makeup for you, she does it because she likes it and you can't tell her otherwise.
I can think whatever I want, my opinion is my opinion.
It's your opinion but it's her body. You can't be controlling of what someone chooses to do with their make over.