The machine can reach 1089p at 30fps. Mario Kart 8 Deluxe will be running at that framerate. It certainly speaks to the graphical quality and depth of the game world that it won’t be able to run at 1080p at 30fps though.
There are a million different factors that determine the resolution and framerate of a game. It’s not like “1080p and 30fps” are just two checkboxes that you can tick. You can run Pac-Man at 1080p, but that doesn’t mean it will look better than Breath of the Wild at 900p. What’s the texture resolution? Draw distance? Shadow complexity? Lighting model? Everything is a push and pull, and obviously Nintendo didn’t want to sacrifice graphical quality for a bump in resolution.
And by the way, a lot of Xbox One games only run at 900p resolution, and I don’t think anyone would accuse that system of having “the power of a mobile device.”