down in the mouth vs downcast

down in the mouth is not a noun while downcast is a noun.

Word	Noun	Adjective	Verb	Adverb
down in the mouth	No	Yes	No	No
downcast	Yes	Yes	No	No

As adjectives, down in the mouth and downcast are synonyms defined as:

Other synonyms of down in the mouth include blue, depressed, dispirited, down, downhearted, gloomy, grim, low-spirited, low.

down in the mouth (noun)	downcast (noun)
	a ventilation shaft through which air enters a mine

down in the mouth (adjective)	downcast (adjective)
filled with melancholy and despondency	filled with melancholy and despondency
	directed downward