mimgrim
Smash Hero
You're probably confusing the f-air with the u-air there, because the u-air appears to execute pretty fast.
u-air
While it is highly possible that is her Uair, it could also potentially be her Utilt instead as she could be on that top platform of BF. I just wouldn't go rule as Uair yet as it could possibly be Utilt also.
As for if I will main Rosalina. I have no idea who I am maining until the game comes out and I get to play all the characters and find out which one's playstyle fits me/I like the best.