Mario Doesn't Host Mario Party
I'm real tired of folks using this assumption to mischaracterize Mario as a Diet Wario.
Mario is a blue-collar plumber. That's his WHOLE character design. He's a humble man who wears his work uniform even when visiting royalty, and lacks the resources or the showmanship necessary to host giant go-kart competitions named after himself. He's also reactive, not proactive. We never see him initiate anything: if Peach doesn't invite him for cake or Bowser doesn't invade, he's content living a peaceful life with Luigi. These are all REALLY important aspects of Mario's character, because without them he's almost indistinguishable from Wario. Wario is the proactive seeker of fortune and fame who puts his name on everything. If Mario ALSO does that, then Wario has no reason to exist.
A good rule of thumb: if it's something Wario would say or do, then Mario WOULDN'T.
The spinoffs aren't parties BY Mario, they're parties FOR Mario, organized and funded by the people he's helped, primarily Princess Peach. You think Bowser would peacefully participate in an event hosted by Mario? Never. But if PEACH was hosting, then of COURSE he'd show up, invited or not.
Lastly: folks often wonder about whether we should consider the spinoff games canon, which is totally fair, because they create a LOT of questions: How/where are they building all these crazy tracks, stadiums, and boards? Does Waluigi really own a giant pinball machine? Is Mount Wario a location or an instruction? My personal headcanon is that (with a few exceptions) they take place in Mario 64 painting worlds. That lets us hand-wave most of the logistics.