Rosalina seems fine against fast characters. Not particularly amazing against them (no more than she normally is against anyone), but not particularly bad against anyone.
IMO, her worst match-up is Villager. She's not terrible against him either, but it's DEFINITELY at least an even match-up. Rosalina can take care of Lloyd Rocket just fine, which is one of Villager's primary tools. But Villager has just as good a recovery as her. And has a few really strong moves. I don't know if the Tree can be negated with Gravitational Pull though, after it's been axed. If it can, it will put things in her favour. Same goes for the Bowling Ball. Both of those attacks can be pocketed by other Villagers, but I have not seen anyone try Rosalina's GP against it.