Name a fed top champion with less impact than a fed Kennen
You can't.
If the enemy mid is fed, it will 100 to 0 Kennen or force Zhonya before he gets in ult range. If the enemy ADC is fed, it will force his Zhonya before he can get close enough to keep them in his ult. No matter how fed Kennen is, he can't do shit to balance out another lane being fed.
This needs to be looked at. It's frustrating to be ahead by 5 kills, but still completely invalidated if the enemy has a fed damage dealer.
Kennen was designed for the state of the game in 2010, and that doesn't carry over to the state of the game in 2016.
. He can't even throw shrooms into a teamfight if someone has Oracles up because they become disabled immediately upon landing. All he does is blind the ADC (assuming he can even reach them) for 2 seconds.