I honestly really hate saying that. Because putting makeup isn't just about being pretty. It takes a lot of time, dedications, practices and efforts. I also see it as a self respect. You can't do much about what you are born with, but there are so much you can do to improve yourself. Saying "You look pretty without makeup" sounds like the said person doesn't appreciate someone's effort. Or in other case, maybe they are really bad at makeup..(e. x. my ex)
i can't imagine a scenario where a girl looks better without makeup. when makeup is used properly, it enhances one's appearance. it's function is to improve appearance. so if a guy is saying a girl looks better without makeup either he's 1. lying / paying a false compliment or 2. hinting that you don't know how to put make up on and are probably putting on too much.
Some girls genuinely don't need any makeup. Some girls over compensate with makeup and look ridiculous. I personally believe it should be subtle and only to enhance what's already there not change the way a women looks.
I will tell the truth if they look better without makeup. I will tell them if they look sexier with light eyeliner or lip gloss (pretty much everyone does) but if they look better with makeup I won't say a damn thing and if pressured I will say I love you with both.
My girlfriend only wears makeup in special occassions. When we first met she wasn't wearing any makeup and I loved how she looked. Now it has been almost 3 years later. I love her with makeup but love her much more without it. She is naturally beautiful to me.