Huck describes the circus as one of the most beautiful sights he's ever seen. He's enthralled by the gracefulness of the performers, the costumes, etc.
It was a powerful fine sight; I never see anything so lovely. And then one by one they got up and stood, and went a-weaving around the ring so gentle and wavy and graceful, the men looking ever so tall and airy and straight, with their heads bobbing and skimming along, away up there under the tent-roof, and every lady's rose-leafy dress flapping soft and silky around her hips, and she looking like the most loveliest parasol.
So Huck experiences a thing of wonder and beauty. Yet the circus is not necessarily "high-brow"; it relies on the suggestion of drunkenness and violence for its biggest thrill. This is the climax of the entire show, and it seems the audience is only really interestd in seeing someone beaten or hurt.
And by and by a drunken man tried to get into the ring—said he wanted to ride; said he could ride as well as anybody that ever was. They argued and tried to keep him out, but he wouldn't listen, and the whole show come to a standstill. Then the people begun to holler at him and make fun of him, and that made him mad, and he begun to rip and tear; so that stirred up the people, and a lot of men begun to pile down off of the benches and swarm towards the ring, saying, “Knock him down! throw him out!” and one or two women begun to scream.
Even after this man reveals himself to be a performer, Huck doesn't get it. Instead of realizing that the ringmaster was in on the gag the whole time, Huck thinks he's been fooled and feels sorry for him, saying he wouldn't want to trade places with him. So, while the circus is visually beautiful, its appeal doesn't rely on intellectual prowess. The same could be said for the king and duke's entertainment, although on a much more vulgar level. Their first night's entertainment consists of a botched Shakespeare routine, which barely draws a crowd, and is unable to satisfy those who do come. So they change to the "Royal Nonesuch" program, which consists of the king dancing around on all fours naked, but with bright paint all over his body. The success of the king & duke's show rests on the pettiness of the townspeople. They know the crowd will be upset, but won't want to let the rest of the town know they've been fooled. So they'll talk up the show, & give the criminals a second full house.