Not quite right, if you mean that she is 'really' i e 'very' kind. You should say 'You are a really kind girl' or 'You are a very kind girl'. As you have written it, it could mean that you didn't believe that she was kind, but now you have changed your mind since she gave you the thoughtful explanation. Word order in English can be flexible but sometimes there are 'nuances' i e subtle differences in meaning.