Potatoes are amazing. They grow easily in a wide variety of soil and produce food which has every nutrient we need except for a complete protein. And, they're a vegetable, so you can tell someone you're eating your vegetables and then go to McDonald's for some frenchy fries
I mean, in fairness, "vegetable" isn't a scientific term at all, so whether potatoes are vegetables (or tubers, or roots, or something else) is totally up for debate.
But they're a hell of a lot more of a vegetable than pizza is!
I've always used the definition of a vegetable being an integral part of a plant, like a root, rather than a fruiting seed pod evolved to be consumed by animals
There are several essential amino acids we need to survive but don't produce in our bodies. No plant has all of those amino acids, but every animal product does. You can get complete plant protein by consuming multiple different plants, so if a vegan has a varied diet they're usually fine.
Cooking potatoes with skin is still somewhat more nutritious than cooking them without (unless you're keeping the water for something else, some use it for stuff like pasta)