swell vs not bad

swell is a noun but not bad is not a noun.

Word	Noun	Adjective	Verb	Adverb
swell	Yes	Yes	Yes	No
not bad	No	Yes	No	No

As adjectives, swell and not bad are synonyms defined as:

Other synonyms of swell include bang-up, bully, corking, cracking, dandy, great, groovy, keen, neat, nifty, old, peachy, slap-up, smashing.

swell (noun)	not bad (noun)
a crescendo followed by a decrescendo
the undulating movement of the surface of the open sea
a rounded elevation (especially one on an ocean floor)
a man who is much concerned with his dress and appearance

swell (adjective)	not bad (adjective)
excellent	excellent

swell (verb)	not bad (verb)
expand abnormally
cause to become swollen
increase in size, magnitude, number, or intensity
come up, as of a liquid
become filled with pride, arrogance, or anger
come up (as of feelings and thoughts, or other ephemeral things)