Humour in fiction is so difficult to get right. It is subjective (and nobody can know for certain what their readers' tastes are exactly here) and of course can range from subtle irony (Jane Austen) to outright farce. But I do know that the best humour in fiction, of whatever kind, always comes from the characters. They feed it. It is not forced on them!
Humour in fiction rings "true" when you feel, as the reader, Character X really would do this/say that and the funny situation/dialogue or what have you comes out of that. So it boils down to how well you've set up your characters and how well you know them as you write them. This doesn't stop you learning more about your characters as your story develops, far from it. I think you can always tell when a writer really does have a grasp on their "people". You will know how they will react most of the time. This grasp increases the more the writer writes - so no shortcuts then! Humour of course so often comes out of people's reactions and the more the writer knows their characters, the more they will be able to "use" this.
Another aspect of fairytales I love is there often is humour in them. Can you imagine the look on the stepmother's and Ugly Sisters' faces when they realise the unknown girl who is going to marry the Prince is Cinderella? You have a wonderful combination of humour and poetic justice there I think!
Image Credit: Image below is from Pixabay.