What is Twain implying about human nature through the advertising for the “Royal Nonesuch”?
What I would say he is implying is that people are total snobs. We like to pretend like we are high class even if we are not. We will go along and like things just because we are supposed to like them -- because people who are cooler than us like them.
You can see this in the posters they make up for the Nonesuch. The major selling point is that these are guys who have supposedly played in Paris and London and such. The ad is basically saying "come check out what the sophisticated people like -- if they liked it, you should too."