expression, noun which may take the genitive case particle 'no', noun
slightly below average; lower middle
Often used to describe someone's appearance as not quite average, but not ugly; can also apply to grades, quality, or performance. The phrase is a fixed expression that functions as a noun or no-adjective.
彼の成績は中の下くらいだ。
His grades are around lower middle.
あの俳優は中の下のルックスだと思う。
I think that actor has slightly below-average looks.