he is decent in a lane swap situation because of being resourceless and can farm with Q if needed and escape from dives with his E's MS buff + flash. The buffs to his W and Ult help him a LOT with his teamfight, and he is in a good spot with current itemization. People aren't picking up Chalice early in mid-lane and not many pick up any early MR meaning he can get off a good amount of damage along with his stun on initiating.
Overall I think he is entirely viable in every elo as long as you have a team that can follow up and not get wrecked in lane (which I cant think of anyone that hard denies Kennen except maybe a really well played Irelia)