Neatly plucked brows

It means exactly what it says, so I would not call it "idiomatic."

AlpheccaStarsI would not call it "idiomatic."

I understood the OP to be asking if the expression was idiomatic in the sense of these definitions: "Peculiar to or characteristic of a given language." or "Characterized by proficient use of idiomatic expressions: a foreigner who speaks idiomatic English."

Anonymous
Is this idiomatic?

How come your eyebrows are so neatly plucked?

Yes.