costumier vs fashion designer

costumier and fashion designer both are nouns.

Word	Noun	Adjective	Verb	Adverb
costumier	Yes	No	No	No
fashion designer	Yes	No	No	No

As nouns, fashion designer is a hypernym of costumier; that is, fashion designer is a word with a broader meaning than costumier:

costumier: someone who designs or supplies costumes (as for a play or masquerade)
fashion designer: someone who designs clothing

Other hypernyms of costumier include clothes designer, couturier, designer.

costumier (noun)	fashion designer (noun)
someone who designs or supplies costumes (as for a play or masquerade)	someone who designs clothing