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.
There's a lot of games that have a sort of boring standard (SC2's 1v1 ladder maps are all same-y, nobody competitive plays Smash with items on, etc.) and the fact that you see this pop up repeatedly in unrelated communities should probably tell you something.
IMO, when you have a game with sufficiently deep mechanics, wackiness tends to obscure that depth, resulting in an experience that's less interesting rather than more. Like if you added Mario Kart abilities to football/soccer and suddenly the winning team was getting regularly blue shelled, I don't think it would improve the experience, even if initially it'd be hilarious.
Now maybe it's still worth it to have "party game" modes in some cases, I'm just saying that these modes might not have legs, so to speak, and thus may not be worth the dev effort if hardly anyone plays them.