When looking at Nintendo's output during the Wii U days, I feel that the vast majority of their titles, aesthetically speaking felt, lighter and softer than normal, even by their standards. Nintendo has always been known to be family friendly and non-conformist, and I love that about them, but with the Wii U, I feel that was taken a bit too far with their games. Like Super Mario 3D Land felt much more colorful and less epic than the Galaxy games. Star Fox Zero felt like a massive step backwards in terms of story and tone. Nintendo Land felt more colorful and saturated compared to Wii Sports, Smash 4 felt brighter and more whimsical etc. There are the rare exceptions with Nintendo publishing Bayo 2, and the Wonderful 101, but you also got controversies like the Xenoblade Chronicles X and TMS censorship debacle. 

Comparatively, Switch era Nintendo games feel much more balanced. BotW is ambient, mature, and quiet. ARMS is cool, flashy and epic, Splatoon 2 feels a lot more anti-establishment and in-your-face than the original game. Super Mario Odyssey brings the grand epic-ness from the Galaxy games back, even throwing in some darkness for good measure. 1-2 Switch and Nintendo Labo feel quirkier and more stylized than the somewhat sterile aesthetic of Nintendo Land, Ultimate returns to the more serious tone and feel of Melee and Brawl. 

Basically, Switch era Nintendo games feel much closer to what the Nintendo style would actually be like with modern hardware, whereas the Wii U era felt more like a cheap imitation of Nintendo's aesthetic. This goes for marketing as well. Wii U tired too hard to pander to young kids, but Switch is a cool and sleek device that can appeal to the whole family, as well as other audiences. 

Well, depending on when you consider the Wii U era to have started, Nintendo was coming off their two most successful pieces of hardware, that were mainly so successful due to their strong casual appeal. I think Nintendo was assuming they could continue to ride that wave. Honestly though, I only started to get that “soft” impression for Wii U about a year after launch. When Nintendo first announced Wii U, it seemed like they were really wanting to make it a console more for the core gamer, offering games like Madden, CoD, AC, ME, Aliens, Darksiders, etc.

I feel Wii U's lack of third party support only amplified the problem. Wii U stopped getting the more serious titles after just a year, and Nintendo was largely doing nothing to combat this. 

The WiiU was a console that was geared to hardcore gamers, but marketed for casuals. They thought everyone who bought a Wii would also get a WiiU, so they made a lot of games that hit a middle ground. Not too casual, but not hardcore either. After the failure of the WiiU for multiple reasons, they geared the Switch mainly for core gamers seeing how it is the core gamers that are always buying new tech, not casuals.

As you said, there were a few hardcore titles like Xenoblade X, BotW, and Bayonetta for the fans that wanted them, but I think they were leaning on 3rd parties to satisfy the needs of the more hardcore audience. When support fell out, so did any chance they had for success.

This time around things are different. AAA games are still missing from 3rd parties sure, but Indies and AA games more than satisfy the itch for 3rd party games for the most part. The big difference is games like BotW, Splatoon 2, Bayo 1,2,3, Prime 4, Smash, Xenoblade, Fire Emblem etc being more geared in depth for the core audience. This give the Switch identity outside of a machine that plays AA games and indies, and makes it a device truly worth having.

Switch is no less aimed at casuals than other Nintendo consoles. It's just that the Switch is versatile enough to appeal to a diverse range of consumers. Half the games you listed, BotW, Splatoon 2, Smash Bros. are as much casual games as they are core games. 

The gameplay mechanics of those games are more catered to core gamers than most WiiU exclusives.  The WiiU had good games, just not ones that core gamers would buy a console for.

Splatoon landed after the WiiU was deemed a failure, making it good for those that already had the system but not so much for those that did not.  They were not going to buy a system for one new IP, so they never really learned how good the game really was.  Fast forward to Switch, a much more successful console largely due to BotW being a launch game, and we have people willing to try Splatoon 2 out on their shiney new console.  Its success speaks volumes.

So you seem to have it wrong.  A game can cater to both, but most WiiU games did not.  The question is not if it has games that only appeal to core gamers, but if it has games that are core enough to attract them.  That is the difference, and Nintendo's focus is clearly more core this time around.

Remember, even games like CoD or Battlefield can appeal to a casual gamer as well.  They are easier to please than core gamers so just because a casual can enjoy Smash just as much as a core gamer, that does not make it any less a core game.

Yeah, I don't really buy that. While it is true that games can be for both casual and hardcore gamers, I'd say it's more the case of Nintendo doing a better job balancing for different skill levels this time. That doesn't mean they're mostly focusing on hardcore gamers though. Come on, your going to sit here and tell me that the decision to make 2 candy-colored detachable motion controllers that can be shared for multiplayer or cardboard construction toys for children weren't made with casual gamers in mind? 

I will agree on Super Mario 3D Land. I finished that game on my nephew's Wii U and I was terribly underwhelmed by the game. Didn't feel like a proper big console Mario in the vein of Mario 64 and the Galaxy games.

I never said they gave up on casuals, I said their focus is clearly more core this time around.

Breath of the Wild (core)

Mario Kart (both)

Splatoon 2 (both, but more core than casual)

Smash (both, I believe more core will play this though)

Xenoblade 2 (core)

Fire Emblem (core)

Metroid Prime 4 (core)

Bayo 3 (core)

Octopath Traveler (core)

Pokemon Lets Go (casual??????  Being a remake of yellow I still do not know where this falls, but lets call casual for go mech)

Core Pokemon game next year (core)

Labo (casual)

Go Vacation (casual)

It seems pretty clear cut to me.  There is little doubt that Nintendo is focusing more on core than casual this time around.  Convince me otherwise....

Many of those games have been on other Nintendo consoles before, even those considered "Casual" like Wii, Wii U, and DS. Splatoon and Smash are much more equal in regards to casual vs. core balance, and I'd even argue that BotW is one of the most casual friendly open world games ever. In the end, it's more about Nintendo learning that you can make a game more accessible to casuals without dumbing it down to the point of insult, something they struggled with in the later Wii and Wii U days.