Actually Splatoon was a shoe-in as it was the real major success of the Wii U.
But I think you might be right. Golden Sun might actually be the number one Nintendo owned IP in the ballot as most of the other fighters are from the existing IPs.
This might give Golden Sun the edge as Sakurai had a massive new-IP drive in Brawl and Wii U. At least 40% (50% excluding echoes) of the newcomers were from new/unrepresented IPs. Sakurai has ranged from 23.08% to 83.33% (including echoes) regarding % of newcomers being a new IP. Ultimate has already surpassed the minimum with Splatoon and Castlevania.
And if you include newcomers with brand new movesets only (no echoes), that % ranges from 42.86% to 90.91%.
Oh I finished my statistics hence the %. I have a bit more to say as well