From first hand experience, the answer is undoubtedly yes. It's embarrassing.
I found it odd that Jett Rocket had more slowdowns than usual when I played it on my Wii U. My fears were confirmed when I tested Super Mario Galaxy 2 extensively. About the only notable slowdown the game had on the Wii was in the Bowser level in world 4 during the brief part where Mario flies towards the camera during the middle of the level. On the Wii U, however, are more slowdowns. The most memorable instance is in the Gigagoomba galaxy right after Mario flies through the big pipe. There's some irritating slowdown when making your way across the triangle platforms.
The conclusion is obvious: The Wii U is indeed less powerful than the Wii, because Wii games run worse on it.
This shocking revelation almost made me consider a non-Nintendo console, but Sony has already confirmed that the PS4 won't be powerful enough to run the games of its predecessor (and thus features even more embarrassing hardware than the Wii U, for a higher price no less) and Microsoft, based on rumors, seems to be hellbent to withdraw from the console business. So I'll have to stick with my Wii U due to the lack of choices, although I won't play any Wii U games on it due to the lack of choices.
Legend11 correctly predicted that GTA IV (360+PS3) would outsell SSBB. I was wrong.
Biased reviews are a pleasure to read. Just my opinion, of course.