The idea that better looking people are more successful is not new. The theory is called "Lookism." See this news story done by ABC's John Stossel a while back.
I think the idea definitely has merit. While I was in law school, they showed us a film where they did a story on a fictional trial using high-school students. The jury pools, however, did not know that it was fake. The study kept things as close to the same for both trials as possible. They used the same description of the defendants, the same attorneys, the same arguments for both sides, the same rulings on the objections and so on.
For the Defendants, they used an "ordinary" looking defendant and a football quarterback who had been picked Prom King of his school. Guess what the result of the trials was? The ordinary-looking boy was convicted, while the football quarterback was acquitted.
As to the story in the Washington Post: generally speaking, both political parties usually nominate the better looking candidate. Although I wouldn't totally discount the story, I don't think that it's the best explanation for what is happening this time around.