incompetent vs fumbler

incompetent and fumbler both are nouns.

Word	Noun	Adjective	Verb	Adverb
incompetent	Yes	Yes	No	No
fumbler	Yes	No	No	No

As nouns, fumbler is a hyponym of incompetent; that is, fumbler is a word with a more specific, narrower meaning than incompetent:

Other hyponyms of incompetent include blunderer, botcher, bumbler, bungler, butcher, fuckup, sad sack, stumbler, slouch.

incompetent (noun)	fumbler (noun)
someone who is not competent to take effective action	someone who makes mistakes because of incompetence

incompetent (adjective)	fumbler (adjective)
not qualified or suited for a purpose
legally not qualified or sufficient
not doing a good job
not meeting requirements
showing lack of skill or aptitude