Obviously she’s not popular to the extent of the big 6. But if we’re going to the extent of including Daisy, Toadette, Kamek, King Boo, Captain Toad, Bowser Jr, the Koopalings, and Poochy (of all characters), the absence of Rosalina and ESPECIALLY Wario sticks out like a sore thumb. If you’re a Mario character that’s popular enough to be playable in Smash Bros, you should probably be in this land.
Daisy has an advantage due to age, but Rosalina has had a much higher impact with her appearances. I suppose if you stopped playing around GameCube, Daisy would be more iconic?