As of now Kennen mid doesn't exist.
His ap damage is complete weak compared to the other mids. It's so low to the point where people are making ad Kennen viable as a substitute. Only role he's played is in the top lane a lane where even champions like Nunu can be safe to scale. Most champs are getting decent buffs like Fizz, Lux, Kass are getting buffs that make them more viable in lane.
), but there's no reason to struggle with him mid when he can easily go top and pressure the enemy top laner.