Mario Party long predates the "Wii era" and has its own established fanbase, it's performing on the Switch in line with other popular Nintendo franchises on Switch (ie: Kirby on Switch has higher than usual sales for a Kirby game, Zelda does, Mario Odyssey does for a 3D Mario, etc.).
Switch isn't ever going to be a "casual" system the way the Wii or to a lesser degree the DS were. It is what it is, a more traditional game system that has the differentiating factor in its functional design (it can take the same game from the TV and make it portable or vice versa) and that's all it needs. If you're sitting around thinking that the Switch is suddenly going to become a very different type of platform it terms of it audience appeal, I think you'll be sitting there waiting for a long ass time.