'round shoulders' is correct and usable in written English.
You can use it to describe someone's physical appearance. For example, "She had a petite frame and round shoulders."
She sports saggy leggings, an estuary accent and round shoulders.
Her low-cut bodice emphasised her round shoulders and her high firm breasts.
For the rest of the day, he found himself looking over at Hina, studying her small, round shoulders.
"You seen Be Be, yet?" an elevator boy with round shoulders and watery eyes asked me as I was going back down.
So does the new collection, with languid day suits in jewel tones, some with soft round shoulders and others reflecting the pointed origami shapes of the season.
Her jackets, this season, had a softer, round shoulder, basically easy basics with a bit of difference.
But these days, a London broil can be cut from several parts of a steer (including top round, bottom round, shoulder, sirloin), and grilling can be the method of choice.
In this section, the round shoulder look preferred in the late 19th century -- it made those bustles look bigger and was achieved with straps attached to corsets -- is followed by the jutting padded shoulders of women's suits by Gilbert Adrian and Elsa Schiaparelli from the 1940's.
There is a hunting rifle clipped onto a rack across the dash of the open Land Rover but Nico, our guide, would have no time to reach it, no time to chamber a round, shoulder the butt and fire.
We've both got slightly rounded shoulders and pointy noses.
