downcast vs down in the mouth

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

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

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

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

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

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