also what kind we learn from the good and evil in stardust?

Obviously, good triumph bad. Good people= good thing, vice verse.

but what else?!

It's not meant to be about much else.
It's a fairy tale for adults and is, by definition, a simple story.
Its good luck that he even included a sex scene( book only)! A very good one even.
Anyway The conflict Good vs Evil is a simple one and as been explored since time immemorial.
Try to think about the story as Tristan's journey.

The most I can think is that evil is done for many different reasons and passionless evil is the worse of all!