It's not an idiom because you can figure it out. It's a childish rhyme meaning someone who is not pretty.
It's not an idiom - it means just what it says. Something took "no" time to come about. It's an exaggeration, but the meaning is plain.
Plain Jane - 2010 is rated/received certificates of: USA:R
Plain Jane - 2004 is rated/received certificates of: Ireland:12
Plain Jane - 2010 Wallflower Jane 1-4 was released on: USA: 18 August 2010
Plain Jane - 2010 No Risk Jane 1-2 was released on: USA: 4 August 2010
Plain Jane - 2010 Conservative Jane 1-5 was released on: USA: 25 August 2010
Plain Jane - 2010 Jane Plus 1-6 was released on: USA: 1 September 2010
Plain Jane - 2010 Do Over Jane 1-3 was released on: USA: 11 August 2010
The cast of Plain Jane - 1913 includes: Jane Gail as Plain Jane Matt Moore as Shelley - the Artist Irene Wallace as Miss Goelet - the Socialite William Welsh as Count von Sternberg
Plain Jane - 2010 Friend Zone Jane 1-1 was released on: USA: 28 July 2010
The cast of Plain Jane - 1916 includes: Sabel Johnson as Ethelda Rathbone
The idiom "as plain as day" means that something is very clear and easy to see or understand. Day and daylight are things that are common in daily life and daylight helps a person to see clearly.