I've always found it odd how people undermine Mario spinoffs in the context of speculation, to put down characters like Daisy or Waluigi as less significant "spinoff fodder" and whatnot. I think the idea that Smash is only concerned about significance within the main series is completely wrong.
What a lot of people seem to ignore is just how significant Mario's spinoffs are to its brand in general. Mario Kart has pretty much transcended the main series in popularity and notoriety. Mario sports games and Mario Party are extremely popular in their own right and do just about as well as some of your main series titles too. To your average gamer, Daisy and Waluigi are part of the main cast of Mario characters.
And from what I've gathered from Nintendo themselves, they seem to feel the same way. Toward Daisy at least, probably to give their young female fans another girl to identify with but Daisy has a major presence at Super Nintendo World in addition to more frequent appearances in stuff like Super Mario Run and now Smash Bros. I've never understood what people have against Daisy specifically, she's built herself up to be one of the most prominent characters in the series despite Nintendo keeping her out of the "main series" games.
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Rosalina on the other hand still does manage to show up in spinoffs pretty consistently, but has fallen quite a bit from her time in the spotlight back during the Wii and Wii U era. I still think she's a good addition to Smash, but IMO has aged a little bit in that respect. Even though she's been in more main series titles than Daisy, the latter has been more consistently relevant to Nintendo's own marketing and public perception of the franchise. She
does have some stickers / badges whatever at Super Nintendo World too... meanwhile Wario doesn't make an appearance at all. Pretty weird.
Bottom line I think the distinction between Mario's main series titles and his spinoff games is way overblown. Someone like Daisy still manages to be consistently relevant and important by virtue of being in... literally everything else. Meanwhile despite Rosalina's more frequent appearances in "actual" Mario games, she finds herself playing third wheel to Peach and Daisy more often than not.
Poor Toad