The phrase “plain as the nose on one’s face” means: ridiculously obvious; as conspicuous or evident as anything could possibly be.
The comparison must have been known to Shakespeare, who used it ironically in Two Gentlemen of Verona (1591).
The Lady Sylvia had enjoined Valentine, who loves her, “to write some lines to one she loves.”
She affects to be displeased with the result, though Valentine’s servant sees plainly that the one she loves is his master.
When Sylvia leaves, the servant says:
O jest unseen, inscrutable, invisible
As a nose on a man’s face, or a weathercock on a steeple!
My master sues to her; and she hath taught her suitor,
He being her pupil, to become her tutor.
O excellent device! was there ever heard a better,
That my master, being scribe, to himself should write the letter?