As a experienced player I can tell you that nearly every new Champion that is released in LOL is overpowered. It's an easily policy from Riot Games.
First of all they need to sell the Champion, they want to make money. Since people buy overpowered Champions and not normal Champions for RP they make the Champion overpowered. Skins and Bundles sell easier with new, stronger Champions better too.
But there is a second point, most people complaining about the overpowerness of new Champion do not consider. If a champion is normal, as strong as every other Champion. Players will loose their Lane/Game with this Champion because they are inexperienced. This would let into the not playing of normal, new Champions because you would need to play a lot of Games to get even with a player who played 300 Games with a Champion. To compensate this issue new Champions are overpowered in the beginning, but get nerfed after people are used to play them.
Please stop any whine about new, "overpowered" Champions, these Champions are fine as they for the time they are overpowered they will get nerfed. It's a current fixing of overpoweredness and underpoweredness.