na-adjective
Describes a gentle, pleasing roundness of shape, often with a soft or full quality.
この花瓶はまろやかな形をしている。
This vase has a gently rounded shape.
赤ちゃんのまろやかな頬に触れた。
I touched the baby's soft, round cheeks.
na-adjective
Used for flavors, voices, or other sensory qualities that are soft, gentle, and without harshness.
このワインはまろやかな味わいだ。
This wine has a mellow flavor.
彼女のまろやかな声に癒された。
I was soothed by her mellow voice.
Derived from the classical adjective まろし (round, plump) with the suffix やか. The kanji 円 is associated with roundness, but the word is now almost always written in kana.