It's more of a tagline, not a subtitle.
Whatever you want to call it, it's there and naming a new game in the series ''Animal Crossing: Population Growing'' would only make it more confusing. And I don't really get how that would be a good thing.
Does your username perchance reveal some nice initials...? XD
I'd like to call it Animal Crossing: Population Growing, purely to get people to not call the GameCube game that.
Town Founder could always be a placeholder name if anything.... I mean it's not unheard of the NA/EU audience to think something is absolutely going to be called one thing, only for the names to change later. Town Founder may be a direct translation of the japanese name currently, and the 1:1 translations never sound right. It would be nice if the game focused on building up the town, but I hope they don't put it to print with this name.
Wasn't New Leaf supposed to be called Jump Out at first? I mean when it was released in Japan only and when did not have an official name yet?
But anyway, I do not care at all about the name. As long as the game is awesome and great and marvellous, I'll be satisfied.