They do look pretty similar.
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The average smash fan is probably just going to refer to her as the blue peach or blue princess.
Do you realize what the problem with this image is? If you actually removed the dresses and hair, they would actually look more dissimilar than they do in that picture, because the dresses obscure their figures pretty heavily. Rosalina is more slender than Peach and Daisy, especially across the shoulders, has a smaller cup size, a smaller torso, and longer legs. She also has shorter arms, which I guess makes since with the smaller torso. But the dresses actually obscure a lot of the differences. Anyway, I have no trouble telling the three characters apart, even in silhouette, but whatever, this argument is stupid.