Splatoon is the 6th best selling Wii U title so far, and seems likely to overtake most of the ones above it, which are between 6 months to 2.5 years older. It represents 5% of all Wii U games sold to date, and has reportedly sold faster in Japan than anything since Wii Sports (2006).

Sounds quite successful to me, though of course it's still only a bit over a year old so who knows if it'll last.