Currently on MSO 145 questions are tagged with fun tag. They are very funny and I love to read them.
Many of questions are old. So currently is fun allowed on MSO?
If yes, what are etiquettes to write fun questions?
Fun is certainly allowed on MSO, but only if it is in context of an on-topic, constructive question.
Basically, fun-for-fun-sake is non-constructive and off-topic.
I've been known to use humour from time to time. Really! I know, I usually sound like a serious dolt, but I have not been above tongue-in-cheek language in posts:
Note that the above are mostly actual on-topic questions and answers, I kept the fun to side-jokes instead. Most of the fun here on MSO we keep fun to the comments.
Using humour to improve the readability of a post or to help make your point while writing a serious question or a serious answer is acceptable, and if done well can be given a fair amount of latitude, but asking 'funny' questions for the sake of it is frowned upon on the main Stack Exchange sites, at least.