A lot of the WarioWare characters would be really easy to introduce into Mario multiplayer games - casual and younger fans who only really care about Kart and Party likely wouldn't be confused by the employees popping up out of nowhere because of how self-explanatory their designs tend to be. Like, imagine you're a kid, you don't even know that Wario has his own games, and you just got the new Mario Kart for your 7th birthday. You boot it up to see all your favourite Mario heroes and villains are back - but there's also 3 new characters:
You just glance at them and you know exactly what their main quirks are and what mood they convey - you then tell all your friends at school who haven't got the game yet that the new characters are a disco dancer, a cyborg scientist, and a witch girl and they'll likely either draw up similar images in their mind or, if they're more in the loop on the game/played WarioWare, know exactly who you're talking about.
As much as I love a lot of the RPG characters and want to see them in major Mario games it's a bit harder to get a feel for their personality or even comprehend some elements of their design than with the Ware cast. A casual fan saying "singing robot from Mario Kart" or "alien with sunglasses from Mario Kart" makes a lot more sense without context and is easier to accurately imagine for a non-fan than say, "one-legged witch from Mario Kart" or "bean prince from Mario Kart".



You just glance at them and you know exactly what their main quirks are and what mood they convey - you then tell all your friends at school who haven't got the game yet that the new characters are a disco dancer, a cyborg scientist, and a witch girl and they'll likely either draw up similar images in their mind or, if they're more in the loop on the game/played WarioWare, know exactly who you're talking about.
As much as I love a lot of the RPG characters and want to see them in major Mario games it's a bit harder to get a feel for their personality or even comprehend some elements of their design than with the Ware cast. A casual fan saying "singing robot from Mario Kart" or "alien with sunglasses from Mario Kart" makes a lot more sense without context and is easier to accurately imagine for a non-fan than say, "one-legged witch from Mario Kart" or "bean prince from Mario Kart".