Dell Conagher
The engineer is engihere
This might sound kinda pretentious, but honestly I don't think it matters if the vibe among most people is that Daisy or Wario are spin off characters, because the truth is more important than public perception. If most people perceive Daisy or Wario to be spin off characters, then most people are wrong. I'm 100% open to changing my mind if someone can convince me otherwise, but that's how I see it.That is correct: technically Daisy is not in the same group as RPG characters, but there is a vibe by many people who thinks that she is a spinoff character because of how frequently she appears there. A bit similar to Wario to an extent: he's introduced in a Super Mario platformer but he's more common in spinoffs (though he's got the benefit of appearing in his own games, so he lacked the spinoff stigma that Daisy and Waluigi are frequently accused of).
Because of how often she is accused of appearing in spinoffs, there is a vibe by some fans that she should stay there, and perhaps relatedly, the Mario Awards even labeled Daisy as a supporting character. More or less admitting that she is not as "main" as the major characters (where Toadette, Kamek, and Diddy Kong are included, the former even first appearing in a spinoff). Though I should point out that I disagree with the opinion (that a character predominantly appearing in spinoffs can only exist there): she can appear in other Super Mario platformers or any other RPG game or a Peach/Luigi game without any issue.
The gist of my post isn't that I feel that there is no downside for RPG characters to appear there. But it's that a less-frequent character can still appear in more games because they don't necessarily have the imaginary baggage that makes them somehow forbidden. Hence I mentioned how it's a "similar feeling" to the one above.
Thank you for reading.
However, I will concede that the definition of mainline Mario characters isn't very easily definable. Defining anything as "mainline" in this franchise is hell. You're absolutely correct that Toadette for example was introduced in a spin off but clearly became a "mainline" character afterwards. Perhaps the definition could be expanded to characters created in-house at Nintendo, including ones created in spin offs? Double Dash was made by Nintendo EAD, which was the team behind mainline Mario (before they merged with SPD to create EPD). So I think a good definition would be that any Mario character created in-house at Nintendo is considered a part of the main cast, regardless of what that Mario game is.
Going by this definition, I do not consider Waluigi to be a mainline character. He was created by a 3rd party company so I don't think he counts, even if he is prevalent in the spin offs.
I feel that there needs to be a distinction between "mainline characters" and "spin off characters" because that helps explain why Nintendo uses certain characters in the way that they do. I think the folks at Nintendo generally prefer to work with the characters they created, and not ones created by other people. This explains why they use Mario, Luigi, Peach, and Bowser for practically every mainline game, while they hardly ever (if at all) use characters like Waluigi, Fawful, or Dimentio.
Spin off characters appearing in Nintendo-developed Mario games aren't an impossibility of course. Goomboss is a Paper Mario (spin off) character who was put in SM64DS and MKDS. Even though those 2 aren't considered mainline by Nintendo, they were still developed by them. My theory as to why this happened is that someone working on those games just really liked Goomboss and wanted to put him in.
And that's what determines who gets to be in the mainline games. There aren't any "rules" against putting characters like Dimentio into Odyssey 2 (or whatever), there just needs to be someone on the team who likes Dimentio and wants to bring him back.