Yeah, I wouldn't doubt that
Splatoon is a killer app for the Wii U (I was actually astonished that it was, as Nintendo's not had much success with new IPs without "Wii" in the name lately), and it definitely helped move systems in Japan, though I have no idea to what extent, just not enough to put it on the same level as the 3DS.
I should also mention that, for some reason, I missed the fact that it was being compared specifically to
Super Smash Bros. for Wii U and thought that it was for all Wii U games.
According to Anime News Network's weekly data, by the way, this is how
Splatoon has done each week up to August 30th overall in Japan.
Splatoon was 1st in Wii U game sales except otherwise noted:
May 25th to May 31st: 1st
June 1st to June 7th: 1st
June 8th to June 14th: 3rd
June 15th to June 23rd: 3rd
June 22nd to June 28th: 6th (5th if both versions of
Fire Emblem Fates counts as one)
June 29th to July 5th: 3rd
July 6th to July 12th: 5th
July 13th to July 19th: 5th (2nd in Wii U overall; beaten by
Yoshi's Woolly World)
July 20th to July 26th: 4th
July 27th to August 2nd: 4th
August 3rd to August 9th: 3rd
August 10th to August 16th: 3rd
August 17th to August 23rd: 4th
August 24th to August 30th: 5th
(Total sales so far: 607,670)
Something to consider is that, looking at the charts,
Mario Kart 8 had consistently outsold
Super Smash Bros. for Wii U every week of that list. In fact, in most of these lists,
Super Smash Bros. for Wii U didn't even crack the top 20. Ever since
Yoshi's Woolly World's debut, it had also been beating
Super Smash Bros. for Wii U every week. It seems that either the 3DS version had been cannibalizing sales of the Wii U version, or Smash Bros. loses steam faster than Mario Kart (though few things can outdo Mario Kart in sheer momentum across a hardware generation). If you want to see the lists themselves, they
begin here. There is a link to the following week for each of these articles.