lowness vs unhappiness

lowness and unhappiness both are nouns.

Word	Noun	Adjective	Verb	Adverb
lowness	Yes	No	No	No
unhappiness	Yes	No	No	No

As nouns, unhappiness is a hypernym of lowness; that is, unhappiness is a word with a broader meaning than lowness:

Other hypernyms of lowness include sadness.

lowness (noun)	unhappiness (noun)
the quality of being low; lacking height	emotions experienced when not in a state of well-being
a low or small degree of any quality (amount or force or temperature etc.)	state characterized by emotions ranging from mild discontentment to deep grief
a feeling of low spirits
a position of inferior status; low in station or rank or fortune or estimation