plainness vs clarity

plainness and clarity both are nouns.

Word	Noun	Adjective	Verb	Adverb
plainness	Yes	No	No	No
clarity	Yes	No	No	No

As nouns, clarity is a hypernym of plainness; that is, clarity is a word with a broader meaning than plainness:

plainness: clarity as a consequence of being perspicuous
clarity: free from obscurity and easy to understand; the comprehensibility of clear expression

Other hypernyms of plainness include clearness, limpidity, lucidity, lucidness, pellucidity.

plainness (noun)	clarity (noun)
the appearance of being plain and unpretentious	the quality of clear water
an appearance that is not attractive or beautiful	free from obscurity and easy to understand; the comprehensibility of clear expression
clarity as a consequence of being perspicuous
the state of being unmixed with other material