Yes sour cream works just as great as a substitute for milk in mac and cheese.
I actually prefer to use sour cream to make the mac and cheese instead of the milk because I find it makes it much creamier and tastes better too.
You can also use Almond Milk or even Yogurt to make mac and cheese with too and it will taste just as good as cooking it with regular milk.
You can also cook mac and cheese without milk if you want too or don't have any milk but the milk, yogurt or sour cream gives it a smoother and creamier taste and texture.