The tablet from “Rottenbeard” reads, “LazyTown should always be...”.
If the people of LazyTown had to be lazy/happy/whatever else, the founder should have written, “LazyTown must be...”.
“Should”, being a verb conjugated in the subjunctive mood, implies a certain contrary nature (or, at the very least, a nature that can be contradicted) in the statement. That is, “LazyTown should always be happy, but that’s just my opinion,”—or something of that nature.
“Should” is far from being an absolute, and it’s ridiculous that no one in the town stopped to consider the rhetorical angle of the situation.
(Then, of course, there’s the whole issue of altering your life based on a statement that is neither sacred nor legally binding, but I can imagine that, had they gone that route, the episode would have been subject to some censorship or outright banning in many places, because I can see that moral being taken as anti-religion or anti-government, even though that’s not even what that alternate moral is about.)