As far as I know, ordinal endings were never superscripted in English typography, even in typesetting, until Microsoft came along and put a feature in Word that did it automatically, by default. God knows why -- maybe a convention mistakenly borrowed from romance languages. I always turn this feature off myself. (Tools > AutoCorrect > AutoFormat As You Type.)
In any case, it's perfectly correct, and for my taste preferable, just to leave ordinal endings in regular type. Under ordinals the Chicago Manual doesn't even mention superscripting as an option, and in one place (17.52) it even specifies that a superscripted ordinal may be changed to regular type.
To the best of my knowledge, the only letter ever used with an ordinal ending in most contexts is lowercase N, as in 'I've told him for the nth time, but he never listens.' In that case 'nth' is just written as a regular word. So, again, it's better NOT to distinguish it in any way.
In a mathematical or scientific context, it might be better to put the variable itself in italics, but for that, you should check a style sheet in your field.