Any champ can go anywhere and do well in the right hands. Faker could take Leona mid, build her full crit, and still dominate any one of us like we weren't even there. Does that mean Rapid Firecannon Leo is now a decent mid laner? No.
Kench has a 45% winrate in top lane with a 0.7% playrate there:
http://champion.gg/champion/TahmKench/Top
Those numbers are so terrible they make Mordekaiser look like Jayce.
He's not even good in the support role:
47% winrate with a 2.18% playrate. When playrates go that low, you expect a higher than 50% winrate since mostly dedicated mains are playing the champion. However in this case both numbers are pathetic.
And they don't list his jungling stats because there wasn't enough games played with him in the jungle to draw statistics from. That's not a good sign.
Don't fool yourself, Kench has been benched. The frog has been clogged. The fish has been squished. The Kermit has been... made into a hermit? I dunno about that last one, but you get the picture.