Why Yankee Doodle.
Reviewer Dave Glazovsky asked why the movie was called 'Yankee Doodle Daffy', understandably expecting a WWII flag-waver actioner with such a title.
The title is a play on 'Yankee Doodle Dandy', the song by multitalented song-and-dance man/composer George M. Cohan, and the title of a biography movie starring James Cagney (doing some amazing hoofing!). So it's really quite apt.