I honestly can't think of another new IP on the Wii U to compare it to besides Wonderful 101.
Zombie U
If Splatoon really had somewhat comparable marketing campaign to Mario Kart I really don't see how around 20k debut week is that good. Sure it's UK and WiiU but still. Well it's not another Bayonetta 2 either though.
For a new ip with the userbase the console has over here, I think it's pretty decent.