As proven by the screenshot below, Mario Tennis Aces: Online Tournament Demo seems to be region locked. While the software inside seems to be the exact same one, the internal IDs are different, so you can have two copies of the demo on your Switch.
What this means is that you won’t be able to play against people from other regions during the demo. (Which is proven by the fact that each region has their own schedule to play the demo.)
So far, it seems the full version will not be region locked, though, as it is possible to open the Japanese eShop listing through the American demo.
(Sushi Striker’s case is a bit different which I’ll explain in a different post.)