dandy vs not bad

dandy is a noun but not bad is not a noun.

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

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

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

dandy (noun)	not bad (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

dandy (adjective)	not bad (adjective)
excellent	excellent