rI don't really know your family dynamic but I would have to suggest that this type of behaviour is learned. Not necessarily by watching or hearing others in the family speaking thusly about people, but sometimes due to criticisms by others to HER.

The fact is, I have never heard of a child (especially at age 7) just pop up and become critical of people, unless it is what she has learned (apple falling from tree scenario) or what she is actively experiencing. (And your comment at the beginning of this post "(her sister's natural hair is soft and silky and brownish)" makes it clear you have noticed an 'imperfection' that causes this child to 'suffer to brush' her own frizzy hair....to a 7 year old, that's quite the criticism especially if it is used to distinguish herself from a sibling who has generally 'soft, brown' hair.)

It is NOT normal in the sense that a child should not express this sort of derision for others in that manner, so openly and regularly. I would suggest looking more closely at her family dynamics (and yours) for a clearer answer to this issue. Once identified, you may want to consider intervention therapy.....before what starts off as a unpleasant attitude, morphs into a full blown personality disorder. IMHO