Of course you can. Its usually only a problem if you seem insincere, or if she doesn't believe it about herself. Use first person pronouns, and if you don't know her very well, stick with 'cute and 'pretty. Personally, if I'm at school wearing no make-up and a guys says You're beautiful, I'm liable to be like pfft, okay, what does this idiot want? Where as a casual I think you're really cute, ya know that? Would make me smile.