It's not so much wrong but as I said anodyne, it doesn't say anything about a person ( other than they are female lol), it's like saying someone is 'nice', a word my English teacher forbade us to use unless in the context of being skilful/precise because it actually doesn't mean anything. Think about it... what sort of day have you had? it was nice. what does that tell you? nothing, not even if the person enjoyed their day, it just sounds polite. However when it's used to describe girls it's actually quite confining because little girls etc are expected to be both 'sweet and nice'. Translated to martial arts it makes teaching females who have been brought up to be 'sweet and nice' difficult, not because they are scared of being punched or hurt but because they've been brought up not to rough around, not to wrestle or play fight and so can't grasp the concept of sparring to hit someone as easily as males. Hitting bags and pads fine, but not people. Nice girls, sweet girls don't hit people, boys aren't supposed to either but it is tolerated because 'boys are boys'. an excuse as we've seen recently which seems acceptable to a lot of people.
What we want for our daughters is for them to be self confident, to feel they can take on the world and to be themselves, nothing of that has to do with being aggressive or 'not nice'. It's about being independent and happy in your skin, it's about having a can do attitude not an 'up yours mate' one. Girls and boys should feel they have their parents approval not for being gender stereotypes but for being themselves.
Being able to defend yourself when attacked isn't about being aggressive and screaming, it's about being confident through training that you can take a punch or two, not freeze, can respond appropriately ( and knowing what that appropriate action is) and can survive. Training yourself to be aggressive isn't the answer. Training yourself to be confident and trust yourself is. That and the proper martial arts training for you.