shoot one's mouth off vs utter

shoot one's mouth off is not an adjective while utter is an adjective.

Word	Noun	Adjective	Verb	Adverb
shoot one's mouth off	No	No	Yes	No
utter	No	Yes	Yes	No

As verbs, utter is a hypernym of shoot one's mouth off; that is, utter is a word with a broader meaning than shoot one's mouth off:

Other hypernyms of shoot one's mouth off include mouth, speak, talk, verbalise, verbalize.

shoot one's mouth off (adjective)	utter (adjective)
	complete and absolute
	without qualification; used informally as (often pejorative) intensifiers

shoot one's mouth off (verb)	utter (verb)
speak spontaneously and without restraint	express in speech
	articulate; either verbally or with a cry, shout, or noise
	express audibly; utter sounds (not necessarily words)
	put into circulation