The phrase "thin mouth" is correct and can be used in written English.
It is a descriptive phrase that is often used to describe someone's physical appearance or a certain expression on their face. Example: She had a thin mouth that gave away her true feelings even when she tried to hide them.
His narrow face, with its close-set eyes and thin mouth, seems closed in on itself.
Hinton, who is 69 years old, has the kind, lean, English-looking face of the Big Friendly Giant, with a thin mouth, big ears, and a proud nose.
The thin mouth opens and the chubby oracle speaks: Obey not the "logic of the mind but the logic of the eye".
The thick hair, the deep seams on either side of the thin mouth, the – frankly – incredibly stupid little moustache lying along the top of his upper lip like a bit of edged furze, the omnipresent cigarette.
His hair was cropped short, military style, above a big forehead that was permanently furrowed, shaggy gray eyebrows, wide-set blue eyes, a pointed nose, and a thin mouth with slightly protruding front teeth.
The prose is beautifully alert and poised: "his thin mouth already moving in the curly shape of wit, as if he were rolling a small, bitter lozenge on his tongue".
The philosophers saw it as a sign of folly, "like an ass", while those with especially thin mouths were thought to be proud, like lions.
Bullheads have thin mouths, sunnies even smaller.
THE little ladybug has a downcast look, her blue eyes nearly teary, her pencil-thin mouth in deep frown.
Here the talent is a Tell It Like It Is retail consultant named Mary Portas, with a razor-thin mouth that never seems to shut and a severely angular haircut that says, "I look fabulous, and you look like a big, bulbous turnip".
Lievens paints his lips full and young; Rembrandt always gives himself the thin, hardened mouth of an older man.
