I love this episode of the novel! We need to remember first and foremost that the whole section with the Duke and the King give us a number of examples that tie in with one of the key themes in the story - the conflict between nature and civilisation. While Jim and Huck are together by themselves going down the river, life is great for them, and we are given a number of examples expressing the tranquillity and ease of their life. However, it is when they have contact with civilisation that problems begin, and through the various scams of the Duke and the King, Huck is presented with civilisation in its worst form.
I would say in response to your question that it is very difficult to pick out who is most victimised, because the townspeople who go to the first showing, who we might naturally think are the most victimised, actually are so ashamed of the way that they had been "sold" or tricked by the Duke and the King that they willingly go along with the trick and fool others into going so they won't look bad:
"Hold on! Just a word, gentlemen." They stopped to listen. "We are sold - mighty badly sold. But we don't want to be the laughing stock of this whole town, I reckon, and never hear the last of this thing as long as we live. No. What we want is to go out of her quiet, and talk this show up, and sell the rest of the town! Then we'll all be in the same boat."
The villagers themselves seem to be worse than the Duke and the King therefore - they agree to lie about the quality of the performance in order to convince others to attend so that they will not be laughed at.
Therefore this amusing episode does not easily give us a victim - the villagers who are fooled show questionable morals in the extreme in agreeing to fool others.