Well, should fairytales ever be funny? I suppose your answer to that might be based on what you believe fairytales are for. Many, of course, give moral messages (e.g. never be unkind to an old person. In the fairytale they often turn out to be someone important and/or magical in disguise. Crossing them or being unkind is never a good move!). Other fairytales can act as warnings. Some, naturally, can do both.
My own view is where a story (of any kind) can be funny and it gets something across better that way, then do so. Truth is often more palatable when sweetened a bit and making someone laugh or smile as they take in the more serious point behind the gag does make accepting that truth easier. I've never seen anything wrong in "just" writing for laughs. It's harder to do than it sounds but I've found the most memorable speeches/sermons/stories have all contained at least some element of humour. Humour makes it easier to remember and can be a useful tool for a writer.
Humour should never be forced. It should arise naturally out of your characters and the situations you put them in. I don't think you can fake something to be funny. It either is or isn't. There are topics which are not suitable for humour at all (abuse is the obvious one for me) but a fairytale which is trying to "promote" being kind, for example, could use humour to make more of an impact on a reader.
I love reading humorous prose. I find it a tonic for the soul. Given stories reflect life, and life should have humour in it, so should our tales! But it works best when it suits the material. You can't force humour into a story where tragedy is more appropriate. Nobody went to see Romeo and Juliet for laughs for example.
Fairytales have great scope for flexibility. You have magical creatures and where there is magic, there is always the potential for it to go wrong. The consequences can be funny. Think of The Sorcerer's Apprentice for example.
Happy - and fun - writing!
Image Credit: Pixabay.