Nope :L
Causing Nightmares is specifacally the effect Darkrai has on humans- like other fairies than cause infatuation, enchantment, misdirection, illusion- I'm looking at it as a tangential take on the abilities of a Fairy, not a literal one.
Well, tell me what YOU think Fairy is/means etc. then with examples of pokemon already revealed with the type
Depends on how broadly you take 'Fairy'- plus I'm not saying Darkrai is based specifically on any kind of Fairy- depends on what you say the bogeyman is- Fairy is not an objective term- could you say the bogey man is a Fairy? Maybe. I would. I could also imagine it getting Fairy typed moves like Moonblast etc.
I see Darkrai as having as similar an amount of claim to being a Fairy as Xerneas is. I personally see reasons, but they may not, evidently, be shared by everyone. Besides I don't think that they are going to change Darkrai anyway so I am arguing a dead case.
I would also have liked to see Absol gaining Fairy type- probably another most people wouldn't agree with.