She's too cheap, a plague that wants to destroy humanity and replace it with brainless zombies a.k.a. Rosalina mains.
Y'know, there's always something to claim as lame, bad-designed etc. and there's always such a claim in basically, every game that implies going against another and is asymmetrical in that regard, and that not only is a bad habit because of the "assuming that to every complain is going to correspond an automatic answer that isn't yours", but as well because such toughts easily snowball out of control.
It's the case with Rosalina's Uair, as calling a move wich has 13 frames of endlag (wich only matters if you are still above her when the move whiffs) is quite unappropriate, most non-super floaty match-ups draw the line of fishing for Uairs kills simply in virtue of this, as well as her air speed usually not being enough to follow-up on whiff.