"We've got another game tomorrow!" say the other children. …

"We've got another game tomorrow!" say the other children. "See you then!"

"Thanks!" says Stanley. "But first I've got to practice the piano." Of course, he did not say that he won't play Baseball with them, so it appears that he doesn't really hate baseball. It appears that he succeeded in getting accepted by the other children, which is what he really wants!

The children watch mystified as Stanley walks off. THE END.

Having gone through the basic narrative, I will now go back and attempt to resolve the contradictions I have noted in the narrative. In particular,

The first two stories are about a misunderstood monster who uses deception to become accepted by societies that hate them, but the guide interprets them as meaning that it's ok to just be yourself.
The third story is about a moral transformation but the guide interprets it as being about a change in self-image.
Stanley says he hates baseball, but he still wants to play baseball at the end of the story.

One may be prone to seeing these contradictions as laziness on the part of the producers, who perhaps licensed three cheap cartoons and attempted to put them together into an overall narrative that they don't really fit into.

However, contradictions are also a sign of covert manipulation. It is impossible to present an illusion as reality in a way that is fully consistent. Thus, a careful inspection of a deception will always reveal contradictions. If we look at Misunderstood Monsters not as a story about a misunderstood monster who becomes human, but rather as a story about a normal kid who is taught to become a narcissist without knowing what was happening to him, then the story becomes self-consistent.