As adjectives the difference between miffy and niffy
is that miffy is easily irritated while niffy is (colloquial) having a bad smell.
niffy
English
Adjective
(er)
(colloquial) Having a bad smell.
Synonyms
* (having a bad smell ):
** (standard ): fetid, stinking
** (colloquial/slang ): pongy, stinky, whiffy