mentally retarded vs people

mentally retarded and people both are nouns.

Word	Noun	Adjective	Verb	Adverb
mentally retarded	Yes	No	No	No
people	Yes	No	Yes	No

As nouns, people is a hypernym of mentally retarded; that is, people is a word with a broader meaning than mentally retarded:

mentally retarded: people collectively who are mentally retarded
people: (plural) any group of human beings (men or women or children) collectively

mentally retarded (noun)	people (noun)
people collectively who are mentally retarded	(plural) any group of human beings (men or women or children) collectively
	the body of citizens of a state or country
	members of a family line
	the common people generally

mentally retarded (verb)	people (verb)
	fill with people
	furnish with people