I admit the narrator would have a lot less of a story to tell if Emma had her fancy under better control, so perhaps the narrator may approve of it in that context. I don't think she is trying to convey an idea that making up stories about people and then asserting them as fact to their subjects ("There can be no doubt of your being a gentleman's daughter" to Harriet in Chapter 4, when the only facts Emma has about "Mr Smith" are that he has money and a sense of responsibility, neither factors exclusive to the gentry) or sharing them with comparative strangers, as in Chapter 26, is an endearing trait.