"Plain looking" can be true or false? By what standard? It's also an opinion that the way a person acts is due to demographics. tl;dr: neither true nor false, just a belief, a value judgement.
Some people in this world would definitely say that it's not sexist to grade women on appearance, and in fact it is a value judgement held in positive regard in many parts of the world. That there is a class distinction that implies standards of behavior follows from this.
