Those are pretty bad examples. Mario Kart and Smash are literally some of the top selling games of all time. If their "wacky ways" are played and enjoyed by 10% of players, that's still millions of sales.
Anyway that's why I also used SC2 as an example. Blizzard's attempts to add more wackiness/variety to the 1v1 ladder maps have generally been poorly received.
Now, SC2's co-op mode has a bunch of wackiness and did great, but note that this is very VERY separate from standard ladder/melee games. It's not just "oh we changed a few rules" or "we took the base game and added wacky items" it's a much more fundamental rework than that, it's an entirely new 'vertical' for the game. And yeah, that kind of thing could probably work for a game like Rocket League.
If your primary audience is casual, like in the case of Nintendo games, then it makes all the sense in the world.
Obviously you could argue that Psyonix ought to be appealing to a casual audience. That's a separate conversation, and I'd argue that RL is a fundamentally bad fit for that sort of game, but given what they have been going for thus far, not introducing crazier modes makes a lot of sense.