Most successful first-party IP = Yes. Based on the latest sales data, Splatoon has outsold every new first-party IP (e.g. Bloodborne, Driveclub) and in some cases, Splatoon has outsold old and established properties (e.g. Killzone: Shadowfall, Halo 5) as well. Splatoon makes, without a doubt, the best case for Nintendo to invest in new IP rather than old and established properties.
Best new first-party IP = No. Based on review aggregate websites, Bloodborne has outscored every new first-party IP (e.g. Splatoon, Wonderful 101). Bloodborne is, thus far, the best new first-party IP of the generation.