You may not like when she wears a crapload of makeup or when she wears mini skirts and crop tops but she only does it for one or two reasons.
Reason one: she thinks that you do in fact like it and wants to impress you in which case you can just say
I think you're much prettier without makeup.
If you've never seen her without makeup you can say. I wish that you'd let me see you without makeup I bet you're beautiful without it.
If its not the makeup but the clothes that you don't like you can say.
Hon you look nice but I think jeans look a lot better on you.
Reason two: the makeup and skimpy clothes make her feel confident and are a way of expressing her thoughts.
In this case do NOT tell her on any occasion that she looks better in something else. Why? Because she could care less.
What you should do is find a way to show her how you would prefer for her to look for example:
buy her a nice outfit that you would like better
do a boyfriend does my makeup video on youtube this helps because even if you use black eye-shadow as eyeliner she'll have an idea of what you like.
Comment on the things you like most about her style for example: tell her you like when she wears neutral colors on her face or when she wears long dresses.
