Their last two games were cartoons because they were Donkey Kong games. ¬_¬

I wouldn't call the lineup homogenous.  Xenoblade Chronicles 2, Breath of the Wild, Metroid Prime, Fire Emblem.  And hopefully this new Monolith Soft project comes to fruition too.

Depends how those games turn out though. If Prime 4 is stylistically like the first three and Monolith's next project isn't cartoony, then yeah, that'll bring some nice variety. But if every single game they bring out is cartoonish, that will get tiresome and dull.

The question is, if he's so tired of cartoon looking games (and is known to have felt like this for years now), why does he stick to Nintendo?

I mean, what do you clasify as cartoony?  In my mind, I see none of those as "cartoony"  Mario is Cartoony.  Breath of the Wild is stylized and Xenoblade is a mix of stylized and realistic and Fire Emblem has an anime style.  And the concept art of the next Monolith project is very much stylized but not cartoony.  And given the artistic approach in Metroid Samus Returns, I highly, highly doubt Metroid Prime will be cartoony or even overly stylized.

I'm including anime and cel shading under the cartoon banner, so that includes both XB2, Botw, and Fire Emblem.

So long as Prime 4 doesn't follow the trend and features more gritty/"realistic" graphics, then I'll be satisfied. It's like pizza; I love pizza but sometimes I feel like eating a nice curry instead.

I think expanding the definition that far renders it so vague it's basically meaningless.  None of those games look much alike for one.  And XC2 is not cel shaded in any sense of the word.  None of those games look like Mario Kart 8, Mario Odyssey, Mario + Rabbids, Donkey Kong, etc.  And implying they do in even a remote sense by throwing them under the same category is, in my opinion, silly.  Shoot I would argue Arms doesn't even belong in the same categorization.

But including XC2 is the most egregious.  How is it any more cartoony than XC1 or XCX?  Because they altered how they textured character models a bit?  Cause aside from that, it is in no way whatsoever more cartoony than the its predecessors, employing realistic/detailed textures, landscapes, lighting, weather, etc etc etc.  So it most definitely belongs nowhere near that categorization.

Because the characters are anime and anime = cartoon. 

Sure, not every cartoon looks identical, but come on, sometimes I just want to play something that looks gritty/realistic and I don't think that's too much to ask. No system should be completely lacking games in such an important style.

If you are responding to my XC2 paragraph, the characters in XCX and XC1 were also blatantly anime.  So I take it those are also cartoony games?  Samus and other characters have anime facial designs in Metroid Prime 3, is it cartoony?  Twilight Princess characters ARE anime, blatantly.  Take it that is cartoony?  That standard seems really arbitrary and vague since it means a game can be 95% fitting of the non-cartoony categorization but then because 5% or less fit this standard, BAM it's cartoony.  

I think you're line in the sand approach is just inadequate for evaluating the diversity of a lineup.  It's a sliding scale and no, Nintendo doesn't have any games that go all the way to the Call of Duty/Battlefield end of being fully realistic, but they have games spread across the scale very generously.  

None of those games are full-on anime/toon though. They may have some character proportions reminiscent of the style but they have their own separate identity.

Nintendo doesn't need to go as far as Battlefield, but something in the realm of a Metroid Prime or an XCX would be a godsend. Again, it is not a lot to ask for, for a major platform holder to have at least one first party title that at least semi-realistic in style. Nintendo themselves in past generations have always had at least something to fill this niche.