What was the moral of the Nightmare Before Christmas?

Know Thyself”

Jack Skellington was and is the best at Halloween, but he is not very good at Christmas. The whole moral of the story can be found in the song “Poor Jack”

As I was told when I was young by my parents, “The older you get and the more things you try, you’re going to find out that there are a few things you are great at, many things you’re just kind of okay at, and some things, unfortunately, that you’re going to be bad at. So it’s important to try as many things as you can so you can discover what those things that you’re great at are, and then you can be the best possible version of yourself.

So that was Jack. His entire life he hadn’t tried anything other than Halloween. So he tried something else, Christmas. He tried, and failed, but walked away with a better understanding of who he truly was, the best at Halloween.

There're actually several morals.

1. Be yourself. In an effort to better himself, Jack tried to be someone he isn't.

2. Stick to what you know. Jack thought he could do Christmas just as well as Santa, but found that neither he nor any of his fellow Halloween townsfolk actually understood it at all.

3. Listen to reason. Sally knew all along that Jack's plans would fail, and in the end, he realizes that he should have listened to her.