not bad vs dandy

not bad is not a noun while dandy is a noun.

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

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

Other synonyms of not bad include bang-up, bully, corking, cracking, great, groovy, keen, neat, nifty, old, peachy, slap-up, smashing, swell.

not bad (noun)	dandy (noun)
	a sailing vessel with two masts; a small mizzen is aft of the rudderpost
	a man who is much concerned with his dress and appearance

not bad (adjective)	dandy (adjective)
excellent	excellent