If you compare MP 9 and MP 10, MP 10 did way better.
MP 10 came out on the 21st and sold 290k;
MP 9 came out on the 17th and sold 214k.
Even in Europe, it's off to a better start 39.200 (MP 9) vs 56k (MP 10).
Could the Amiibo included in the game be responsible for this?
Unfortunately, Wii U sales aren't good...
20% increase in the first quarter is actually really bad when in 2014 Wii U basically had DK in February and Mario 3D World released in December. Now, Wii U had Kirby and MP 10 and add to that 2 system sellers: MK 8 and Smash.
Even if you take out MK 8 out of the equation, things still look bad.
With the catalog of games Wii U already has, it should be doing better than 20% increase.