George Lucas said he was imitating a common theme in literature where the great and powerful leader is introduced in this way before revealing his true powers. Aparently Jar Jar Binks was handled the same way, but Lucas changed his mind after the backlash. Check YouTube for the Darth Jar Jar theory and you'll see he's not really a bumbling fool. Lucas hurriedly invented Count Duku to replace Jar Jar as the major villain and Dark Side mirror of Yoda. This was supposed to be Jar Jar's role.
Here you go:
https://www.youtube.com/watch?v=PlyRLtAngxo
Btw, the actor who played Jar Jar admitted this was true. :-)