Both of those are at the low end of cooking time * temperature. Mashed potatoes provide essentially zero vitamin C same with many potato soups. https://fdc.nal.usda.gov/fdc-app.html#/food-details/170699/n...

365 calories of raw potatoes have 155% of your daily vitamin C requirement. A 365 calories serving of French fries has ~9% of your daily vitamin C requirements. Of course the cooking oil is part of that difference, but few cooked foods have much vitamin C. Worse many things like rice, bread, or beer provide none.

So you need some aspect of your diet to be a plentiful supply or take a supplement.