I think the reason why Rosalina has certain unfavorable match-ups is mostly due to her attributes, being that she's the fourth lightest character in the game, and also one of the largest, as well, behind only Bowser and Ganondorf. This, however, could be also interpreted as an advantage for her, since it means her hitboxes are also huge, and can reach farther than most characters smaller than her.
I mean, if I were Rosalina, I would certainly try to find a way to get around my clear disadvantages by using them against the opponent, would I not? Due to my size, I would certainly use it to my own advantage, since my attacks reach, again, longer.