I doubt that was the case for this splatfest, honestly. Ketchup is put on just about everything in the Americas and is enjoyed by nearly everyone. Not that mayo is a niche thing here, but ketchup is about as American as a bald eagle eating a cheeseburger.

I'm not surprised by this, honestly. Popularity has always proven to be a double-edged sword in splatfests, where more players on your team also means more low-skill players and more matches played against your own team than the enemy. S2 is a little better with this by making popularity matter more than S1, but it looks like it's still better to side with the large underdog than the mega majority.

(This has been an observation from an individual who is most definitely cursed because they have never once been on the winning side of a splatfest.)