second class vs low quality

second class and low quality both are nouns.

Word	Noun	Adjective	Verb	Adverb
second class	Yes	No	No	Yes
low quality	Yes	No	No	No

As nouns, low quality is a hypernym of second class; that is, low quality is a word with a broader meaning than second class:

Other hypernyms of second class include inferiority.

second class (noun)	low quality (noun)
a class of accommodations on a ship or train or plane that are less expensive than first class accommodations	an inferior quality
not the highest quality in a classification
not the highest rank in a classification

second class (adverb)	low quality (adverb)
by second class conveyance