Like Tracey W I don't think we're supposed to see the Middleton's lack of development as a good thing, but the narrator's view of them with regards to their taste is chiefly Marianne's. We have already heard from the narrator that Sir John is a good hearted man, who wants to assist them in many ways, materially and socially, so we already know he is a good person. To do a 180 on an opinion of him indicates that this is Marianne's view. It's in keeping of her 'rule' that one's taste must coincide with yours in every way to constitute proper taste. She has already shown this with Edward who we know to be a good person as well.