I agree with the many posters who believe the dottering fool approach was in the style of Jimmy Stewart.
Not only the character, but the entire concept of Clint’s parody came from the Jimmy Stewart movie “Harvey.” Here is a scene from that movie:
http://www.youtube.com/watch?v=bKECvqSVWPA
Yes you are correct, the style of Jimmy Stewart and the movie Harvey. I doubt if there are too many of us who actually remember Harvey.