The phrase "a plain face" is correct and usable in written English.
It can be used to describe someone whose facial features are unadorned or not particularly striking.
Example: "Despite her plain face, her warm smile made her incredibly approachable."
Alternatives: "an ordinary face" or "a simple face."
When I saw the collection, I saw how a plain face and pulled-back hair fell into place.
In his 40s, he was a bustling, jovial man with a plain face and a busy and ingenious mind.
"I didn't want to fall into the mom trap of leaving the house in sweats and a plain face, but I don't have a lot of money or time to spend on makeup," she said.
In the opening of Charles Dickens's A Tale of Two Cities she is dismissed in the second paragraph: "There was a king with a large jaw, and a queen with a plain face, on the throne of England".
There he was looked after by a beloved English nanny with a large bosom, a plain face and, on the evidence of photographs of her, unfortunate taste in shoes.
A smile on a plain face can be just as appealing as one on the face of a beauty queen.
Its plumage also is distinct, as the bird has a plainer face and streaked throat.
The corn crake is larger than its closest relative, the African crake, which shares its wintering range; that species is also darker-plumaged, and has a plainer face.
She also had a stocky figure and an arguably plain face, but it was left to her prospective mother-in-law to remark, in 1954, "She's like a little bull!" Of her Oxford years Iris later wrote, "My God, that was a golden time".
Drummond turned to the social worker, whose long straight hair framed a lovely, plain face.
The juvenile common firecrest might be confused with the goldcrest, but usually shows enough face pattern to distinguish it from its relative, which has a very plain face at all ages.
