My personal opion from what you said about her nickname and the fact that your nieces father told her she is perfect is that she probably does not have ADD or ADHD but she is simply spoiled by her parents. I mean no offense to her parents or her by saying this.. Also is your niece an only child? Does she always blame other people for things that she does wrong or is only when there are other children present? Since she is about 7 she probably picked up the hurtful things she says from people that are around her, perhaps her mother, father, Teacher, or TV that she watches. the talking alot and rowdiness are some symptoms of ADD and ADHD however i personally don't think that she does. If her parents or you and your mother feel that she should be screened for ADD/ADHD it isn't a waste of time. But Like you said it does get over-diagnosed in people her age.
I hope everything works out for you,
-Jason