My daughter likes to do that too, but she doesn't think it is the same both ways. Does your DD read upside down as well? I ask because if she can easily read upside down as well she may not really see a problem with mirror writing, whether or not she is doing it on purpose. Is she doing it on purpose? Have you asked her why she does it that way? Does she find it more stimulating to write that way rather than the correct way or is it a mistake she does without realizing it?