No, I mean people genuinely MIX UP the two.
They have a similar colour palette, but that's about it. Rosalina's Ice World is a pretty insignificant coincidence (it's not like she made it with Ice magic or anything, unlike Elsa, who can create things made out of Ice), and "sad past" and "use magic" are too broad to really note them as similarities. Plus, none of these points have anything to do with their personalities, which people try to say are similar.
And the annoying part is that when people see Elsa in Frozen, they don't ever mistake her for Rosalina. It only works the other way around. Even though Rosalina has existed for a much longer time than Disney's Elsa.
EDIT: So I decided to find a screenshot of Rosalina in Smash 4 and compare it to Super Mario Galaxy and 3D World, for those who are curious: