Why would the speaker keep emphasizing a "nice girl?" Is it a common expression?

Native speaker’s answer
Rebecca
Yes, there is a cultural idea that a girl should be "nice." This means polite, friendly, and kind to others. This idea is particularly true for celebrities, who get criticized if they're not a "nice girl" or "nice guy." Taylor Swift is talking about this idea here, the expectation that she should always be nice. Ex: You'll like Julie. She's a really nice girl. Ex: I don't want to be a nice girl anymore. I want to do what I want, not what others tell me to do.