fiddling vs niggling

fiddling and niggling both are adjectives.

Word	Noun	Adjective	Verb	Adverb
fiddling	No	Yes	No	No
niggling	No	Yes	No	No

As adjectives, fiddling and niggling are synonyms defined as:

Other synonyms of fiddling include footling, lilliputian, little, petty, picayune, piddling, piffling, trivial.

fiddling (adjective)	niggling (adjective)
(informal) small and of little importance	(informal) small and of little importance