Would it be mean to tell a girl that I'm not her friend
Why do you want to stop being her friend?
She's so silly. I want her to stop thinking that I am her friend.
It seems like it might be a little more serious than that. If you do not want to be her friend, there is probably more to the story.
I just don't like her. I find her look unattractive.
Picking who you are friends with based on how they look is really superficial. You can't tell what kind of friend someone would be that way. You should try getting to know her before ruling her out as a worthy friend.