With all the circumstantial evidence we have on hand right now, her "chances" are still at a 50fighter/50assist for me, though I have little doubt to say that she's in at some capacity. Though I hope that Sakurai and Team realized that she's best suited as a fighter and not an assist trophy, I'll try not to cry like a little wuss if she's in as an assist trophy.
And yeah, I put chances in quotations because Sakurai and Team already chose their fighter roster technically, they're just finishing up on the final fighters and the final touches on the overall game by this point. Man this is gonna be a rough night, indeed...
Personally, I hope Skull Kid's a boss character, because I would feel that he would feet better as that if we're going the "purist" route of Majora's Mask Skull Kid and not the maybe-maybe not canon of Hyrule Warriors Legends. That and I don't have any good ideas for a moveset for a playable Skull Kid.