Rosalina’s exact origins are shrouded in mystery, though you would be perfectly well forgiven if you found the children’s book on her bookshelf about a little girl who lost her mother and traveled into space only to get adopted by several hundred Lumas and you wondered if the similarities were more than just coincidence.
Nevertheless, Rosalina is a veritable goddess of space, titled Mother of the Cosmos. Her job is raise Lumas into becoming stars, planets, galaxies, universes. Her job is to raise them, and then watch over them, from afar.
Also like, briefly in there, she got directly involved because Bowser crashed through her corner of space dragging Peach behind him and Mario got dropped literally in her lap, so. That was a thing.
As for DWYC, as a goddess tasked with watching over the universe, of course she saw the threat that was coming for it, and of course she needed to do something about it. But it was much bigger than she could handle alone, so she outsourced the job to Alvis and his siblings. She might have chosen differently if she realized it was going to go like this, but she trusts that Alvis knows what he’s doing. If she doesn’t trust that, then what else can she do?