Huh. News to me. This time, it probably is just because she has an amiibo. In other games, it's mostly just because Nintendo cares about her more than the Wario franchise as a whole. And Waluigi. I'm still salty about her continued representation.
I don't even mind Rosalina, I just don't understand why the hell Wario has to be shoved back into the "Crossover Material" corner. He has two great series' under his belt, so do something with it, Nintendo! Wario Land 3D or 5/6 and WarioWare U, please!
I think Rosalina's a fine character, I'm just mad that she gets shoved into games she has no business being in, like Super Mario 3D World, while more deserving characters like Wario get shoved to the sidelines. Seriously, there's a Wario amiibo. He really should be in this one.
Her inclusion in 3D World felt very rushed too, and they didn't make a big deal out of it. No cutscene or anything for unlocking her. Just randomly appearing at the end of that level and an "Oh, you can play as Rosalina!" message after that. She was just sort of...there.