Emerson means that people punish the nonconformist but it doesn't matter because the self-reliant person should stand up against public opinion. He says:
For nonconformity the world whips you with its displeasure. And therefore a man must know how to estimate a sour face. The by-standers look askance on him in the public street or in the friend’s parlour. If this aversation had its origin in contempt and resistance like his own, he might well go home with a sad countenance; but the sour faces of the multitude, like their sweet faces, have no deep cause, but are put on and off as the wind blows and a newspaper directs.
He is saying that the people's sour faces or sweet faces have no real meaning because they come and go like the wind. It is so true. Public opinion is fickle or arbitrary.
Emerson would say it is better to be true to yourself and forget what people think or how they react.