At this point in time, it's obviously Mario Kart 7 since it has had the time to show its ridiculous legs. Let's get back to the subject in 2023 if Splatoon 2 is still selling.
So MK7's legs will be more impressive because it charted 6 years after release even if ends up selling millions fewer than Splatoon 2?
I'm sorry but Mario Odyssey, Mario Kart or Splatoon for example are also kids and women friendly IPs. It's certainly not like the Switch library was only constituted of FPS. Maybe if there is a lack of kids customers for the Switch this could be due to the price being still quite high and the current model appearing way too fragile (Switch Mini anyone? :D) but I doubt it's mainly because of the library.
Mario is a platformer in a colorful cartoonish universe and Kirby is a platformer in a colorful cartoonish universe as well. Yes there are differences but seriously nothing that decisive to attract a totally different audience. Just like the fact that it has a 4 players coop option. I can understand that this option was a big deal for NSMB Wii as this was a new feature for a 2D platformer at the time but seriously that's not that big of a deal really nowadays. Doesn't Rayman even have the same option for example? New features can be important to sell software and hardware if they are totally novel just like the new direction of MH helped it a lot to make it a big system seller but a 4 players coop 2D platformer is nothing so groundbreaking in 2018 that it has to be an instant must have for many.
I don't really think a game needs to be ground breaking in order to bring in new audience to the Switch.
I also don't think that Mario being a platformer makes Kirby less of a draw - Mario ain't exactly cute in the same sense as Kirby while Rayman is a non-entity in Japan.