Apple's goal: Be relevant in gaming outside of mostly F2P mobile games.
I'm not so sure that's true. At least the mobile games part, F2P is bad for them of course. But mobile games for <$5 is the holy grail for them. Or non-F2P and taking their cut for transactions. As we know, financially they're very successful with mobile games of other companies. I'm pretty sure they'll take whatever they get for gaming and that does include more expensive iPad games and also macOS. But that doesn't mean that's where their focus is for "serious gaming". Serious gaming for Apple is where the most money is.
How Nintendo would help Apple achieve the above goal:
1. Nintendo's games would make Apple Arcade 10x more attractive.
2. Nintendo would bring droves of gamers who don't play F2P games into Apple's ecosystem. This would make Apple's gaming platforms more relevant. Overtime, this will snowball and 3rd party developers will release ports on Apple's platforms including iOS, iPadOS, macOS, realityOS, tvOS, etc.
As pointed out by
@ipponrg, Nintendo games are on iOS already. And I wouldn't say these are bad games per se, they're just not what people (including myself) expect from the Nintendo IP... maybe that makes them bad? Considering some of these games are super easy to make, it might make sense:
https://www.gamesindustry.biz/nintendo-mobile-games-have-brought-in-usd1b-in-lifetime-revenue. These games didn't help the iOS market and Nintendo pretty much jumped the ship.
So far Nintendo games didn't help Apple (except for their cut) when it comes to gaming. Which brings me back to my original statement, it would help Apple only if Nintendo were to abandon their own hardware in the future and sell the "full" Mario, Zelda, etc. games for iOS and ATV... exclusively. What are the chances? I guess we'll see hell freeze over and flying pigs before that happens.
However, I'll say that it would be interesting to see Mx in the next Nintendo console... but chances are pretty slim for that as well.