Here are my reasons

1. There are 2 places called Sherbet Land in both Mario Kart 64 and Double Dash (As well as Slam Basketball and the Olympics with Sonic), so those are probably in the same area

2. Both Sherbet Lands have icy lakes, just like Vanilla Lake from Super Mario Kart, as well as the remake of Snow Land from Super Curcuit

3. Merry Mountain in Japan is Merry Merry Mountain, combine that with being a snow-covered mountain with a slide you can go down, it being an alternative version of Cold Cold Mountain from Super Mario 64

4. Both N64 Sherbet Land and Cool Cool Mountain both have another place in the same game with loads of snow (Snowman's Land and Frappe Snowland)

5. Surprisingly, the French translation for GCN Sherbet Land is Sherbet Kingdom, and do you know another ice kingdom in the Mario franchise that just happens to be on an ice/snow filled place and as stated before, has penguins? (Hint: Do you yield?)

I'm not gonna tie EVERY place together, of course the mainline Mario ice/snow stages aren't here, DK Pass and Summit, Rosalina's Ice World, Mount Wario along others aren't here, but I couldn't find a solid reason to tie them together, but still those are a lot of places that seemingly are in the same kingdom

... Yes, I didn't forgot the Ice Kingdom from Odyssey, thing is, while of course they have the snow and ice theme, and the racing areas being similar to Mario Kart, those aren't big enough reasons to put it along the rest, not to mention it creates a paradox because the I put the Ice Kingdom from the movie with them and the Ice Kingdom from Odyssey is clearly a different place, so yeah it's own area, maybe another places like DK Pass and Summit are there or something