I don't know a ton about jesters historically, but in Shakespeare and in contemporary fiction (Robin Hobb's Fitz and the Fool comes to mind), the jester is one of the few characters who says the things that aren't socially acceptable, and since it's a "joke," the criticism isn't cause for offense.
This is a very romantic idea of it. There are always limits, and if you think those limits were the same strictness for the people the court loved as for those they hated or thought beneath them, I have a bridge to sell you.
