Stupid and/or stupidly
Solution 1:
So we see a total of four uses of the word stupid (adjective, adverb, and two nouns). Other words in question are stupidly (adverb), stupidity (noun), and stupidness (noun).
The Proper English uses are stupid (adj.), stupidly (adv.), and stupidity (n.):
adj. ... 1 unintelligent; foolish (a stupid fellow). 2 typical of stupid persons (put in a stupid place). 3 uninteresting or boring. 4 in a state of stupor or lethargy. 5 obtuse; lacking in sensibility. [Derivations:] stu·pid·i·ty /-íditee/ n. (pl. -ies). stu·pid·ly adv.
——The Illustrated Oxford Dictionary, DK Publishing (1998)
Colloquial and slang uses are stupid (adv.) and stupid (n. 1 & 2):
Adverb
stupid (comparative more stupid, superlative most stupid)
(slang) Extremely.
My gear is stupid fly.
[...]
Noun
stupid (countable and uncountable, plural stupids)
A stupid person; a fool.
(colloquial, uncountable) The state or condition of being stupid.
His stupid knows no bounds.
——Wiktionary.com
While I can't find it in many online dictionaries, Wiktionary hits the nail on the head here: The slang adverb stupid means "extremely." The sentence sounds off when applied to verbs, like in "He's driving stupid" (the better phrasing being "He's driving stupidly"). Stupid works better as an adverb for adjectives: "That's stupid cool," "He's stupid bad at driving," or even "He's a stupid good driver."
As for our colloquial nouns, one might call someone a stupid just like you say. It can also sometimes be like a synonym for stupidity. "You can't fix stupid," "The air of stupid in this room is so thick it's almost tangible." (In my town, this one might take a second thought to understand what you're saying.)
As for stupidness vs. stupidity, this was answered here. Being constructed with similarly defined suffices, the two words are synonymous. Stupidity has long-standing acceptance. Stupidness is rarely and endemically preferred. I have no evidence for it being used outside the West Indies.