Saying a "character does not deserve to be in Smash" is not the equivalent of hating them. Objectively, Waluigi doesn't deserve a playable Smash appearance right now, while Rosalina was the last Mario character to seriously deserve a spot. Waluigi has no distinctive personality, he is literally just a copy of Wario, and his appearance is just Wario with all of the fat and muscle taken out, and then put into a stretcher, and changing his mustache to an L shape as opposed to a W shape.
Rosalina, on the other hand, has a lot of details that clearly set her apart from Peach. Her overall body shape is different due to her height, giving Rosalina much stronger and longer looking legs than Peach, slightly thinner hips, and they have slightly different breast sizes (YES, I WENT THERE). Her face is slender and elegant in shape as opposed to Peach's puffy, round, cute face. Her nose is more slender, her eye shape is different, as are her eyelashes. Peach is pure blonde, and Rosalina is platinum blonde. Rosalina has a long, slender neck, Peach's neck is on the short side. Their hairstyles are quite obviously different, their dresses are completely different (Peach's looks regal and fancy, Rosalina's looks like a modest yet elegant nightgown). They have different adornments too. And Peach wears heavier make-up than Rosalina. Rosalina's skin is also paler.
As for character, don't make me go into Rosalina's character. I will go on a tangent. But most people can see that she had a touching backstory (a first for characters within 3D Mario platformers), and quite a bit of character development, as well as having a balanced personality (meaning she was not portrayed as overly happy, but she also was relieved of some of her stress and sadness, preventing her from feeling like a broody character). That's the short version.
And even WITHOUT all of that, nothing will change the fact Rosalina represents the Mario Galaxy series, which are two of the most critically acclaimed games of all time. Waluigi has only ever been in the milked Mario spin-off games, which we get every year. If anything, Waluigi represents a fairly negative side of the franchise.
I don't know how he can have "nothing to offer" when clearly he was never given a chance to have his own game.
Rosalina also never had a game all to herself. Whether or not a character has a game all to their self is not what defines whether or not they have anything to offer. It's what they did in the games they were in. Waluigi just became Wario #2. Rosalina actually was very different within the Mario universe, and there isn't any other character quite like her.
