In a way, everything that happens in this episode is Bart’s fault.
The overall quality of the episode is the producers’ fault, but everything the characters here do is more or less Bart’s fault.
How does this whole thing happen because of Bart? First, word spreads that Principal Skinner is retiring. At a large farewell ceremony, Bart is asked to step forward as a randomly selected student representative to say some final words to Skinner, but Bart uses them to take a final shot with his slingshot.
That sounds par for the course. What isn’t is that Skinner was ready for it. He deflected the shot (it hit Agnes Skinner instead), then had Groundskeeper Willie dump the honey. What honey? The honey that Skinner had set up in a bucket atop the flagpole. Some other crap was added on, and the reveal was Skinner’s wasn’t retiring. It was a ruse to prank Bart for once.
Bart’s in a bad place what with the fact even Mr. Largo is laughing at him, so he does the only thing he and only he would find reasonable and glued clown masks to the faces of every school-related adult he could find while they all slept. The resulting chaos and clown-related fear causes mass hysteria, which is just as well considering Pennywise was apparently in the sewers all along, and even causes problems for OK Go’s newest video. And it causes problems for Krusty, because now clowns and scary and he has to quit. A demoralized, make-up free Krusty can’t do what he always does, and leaves his show.
Now, Bart ends up going to rehab after Marge petitioned the judge, Judge Dowd (guest star Andy Daly), to actually punish Bart for once, and that works such that Bart can’t prank at least Marge anymore. Sure, he goes back to school to apologize, but Willie convinces him to set up a water balloon trap, and Bart almost stopped it when he saw Marge come in, but he’s too late and everyone gets soaked.
As for Krusty, he tries to become a serious actor and joins the cast of a serious play, a knock-off of Death of a Salesman written by the Jon Lovitz-voiced Llewellyn Sinclair. Man, they brought that guy back? Huh. Krusty is even doing well, then on opening night he gets nervous and his attempts to calm himself on stage makes the audience laugh, so he goes back to clowning.
Not amused by that? The ghosts of Rabbi Krustofsky, William Shakespeare, and especially Arthur Miller.
Though I think Miller had other problems beyond whether or not Krusty is a clown.