What is the noun form for "unadorned"? Like "simpleness" for simple.?

Actually, the noun for simple is simplicity.

plain
simple
bare
naked

plain and unvarnished depending on the part of the country you come from. as in the plain truth or the unvarnished truth.

unadornedness

https://en.wiktionary.org/wiki/unadornedness#English

unadornment. But that's not usually used. People are more likely to say "lack of adornment"

Unadorned is an adjective. Any "noun form" of the word would also be an adjective.