It doesn't matter one damn bit whether fashion is art or not. You don't question whether an incredible chef is an artist or not-his cakes are delicious and that's all that matters.
Fashion is also a form of art, and like every kind of art, it has its own way of expression. In other words, if a dress looks better on a thin girl, on a catwalk, during a very specific moment of time and space then it's represented as part of a "fashion Show". It is after all a "Show" and it has to be understood by people that it is a "show" and not real life.
My parents never mentioned anything about fashion in our household, instead we used to talk about literature, theatre, and arts...this is why I have kept a real relation with the Art world, by putting books from the beginning in my shops' windows.