"at least they should do so" What does that mean in the story? "at least they should rule the world"?
ex)One morning, as he was on his way to Holland Park, where the Mertons lived, he dropped in to see a greatgriend of his, Alan Trevor. Trevor was a painter. Indeed, few people are not nowadays. But he was also an artist, and artists are rather rare. Personally he was a strange, rough fellow, with a freckled face and red, rough beard. However, when he took up the brush he was a real master, and his picture were eagerly sought after. He had been very much attracted by Hughie at first, it must be admitted, entierly on account of his personal charm. "The only people a painter should know," he used to say, "are people who are beautiful, people who are an artistic pleasure to look at, and restful to talk to. Men who are well-dressed and women who are lovely rule the world——at least they should do so." However, after he got to know Hughie better, he like him quite as much for his bright, cheerful spirits, and his generous, careless nature, and had asked him to come to his studio whenever he liked.
Yes, it means that well dressed men and lovely women should rule the world.