dress up vs ornament

dress up is not a noun while ornament is a noun.

Word	Noun	Adjective	Verb	Adverb
dress up	No	No	Yes	No
ornament	Yes	No	Yes	No

As verbs, ornament is a hypernym of dress up; that is, ornament is a word with a broader meaning than dress up:

Other hypernyms of dress up include adorn, beautify, decorate, embellish, grace.

dress up (noun)	ornament (noun)
	something used to beautify

dress up (verb)	ornament (verb)
put on special clothes to appear particularly appealing and attractive	make more attractive by adding ornament, colour, etc.
dress in a costume	be an ornament to
dress in a certain manner
make something appear superficially attractive
put a caparison on