It's not an official subtitle therefore the game technically isn't called ACG, but I personally still call it that as it helps specify which AC game I'm talking about.
No, it's not. "Population: Growing!" isn't a title, it's a slogan. Just because it's on the cover doesn't mean it's part of the title. You don't call New Super Mario Bros. Wii "New Super Mario Bros. Wii: Up to 4 Players!", do you? "Up to 4 Players!" is clearly visible on the cover, so by your logic, it should be part of the title, right? Face it, "Population: Growing!" isn't part of the title. It's never called that on anything official, and the spine of the game case (which always has the full title of the game on it) says only "Animal Crossing". If you don't believe me, look here.