incompetent vs stumbler

incompetent and stumbler both are nouns.

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

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

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

incompetent (noun)	stumbler (noun)
someone who is not competent to take effective action	a walker or runner who trips and almost falls
	someone who makes mistakes because of incompetence

incompetent (adjective)	stumbler (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