The mood of the story does not necessarily reflect the mood I'm in when I write the stories! Indeed I often like to write light as literally a kind of light relief when life is on the difficult side. But I also find I can't write light all the time. Sometimes I need what Terry Pratchett once referred to as tragic relief (in my case it tends to border on the criminal/horrific relief - I dread to think what Freud would make of that!).
This, I think, is a good reflection on fairytale writing. Many are grim (especially the original versions). Many have humorous moments (especially when sending up people's vanity, The Emperor's New Clothes is a great example of this).
But what is important is that I write regardless of mood... as should you.