I've beaten X, and I'm in my Y playthrough right now but I've yet to find a single Dedenne. It's worthless so I don't mind much, but the fact that useless rodent has been so elusive to me is aggravating, honestly.
I do know that leading with a Pokemon that has the Static ability can increase the chance of a Dedenne showing up on Route 11 because the ability's out-of-battle-effect is causing Electric-types to appear often.