Sympathy and empathy are very important skills in relating to others successfully. Unfortunately they are also learned skills to a great extent. If a child is not taught to empathize with others not like them and to feel sympathy for those who are hurting, then it will be obvious.
My suggestion is to speak from your heart. You clearly don't want to hurt her feelings and that is admirable. Try to teach her about empathy. (How would you feel? etc) Talk to her about the fact that any of the people situations she makes fun of could just as easily be hers. She could be disabled. She could be poor. She could be different, or whatever else strikes her as funny. Remember that being someone admirable doesn't make you better than anyone else. Try to convince her that she wouldn't like it if it were her. Make her understand walking in someone else's shoes. Also be honest: tell her what she says offends you and you find it immature. I agree with the above post about lack of self esteem. Try to help her find value in herself without putting down others.