In Wario Ware games, it shows Diamond City is where Wario lives, so that's my guess.
That would be my guess too..
Also, I just thought of something.. Why did Wario have to create a company just to make a few coins?
It's not like coins are hard to come by in the Musrhoom KIngdom (or lands surrounding)..
Just jump down the right pipes, jump off the right note blocks, or simply look in the air around you.. Coins!
.. that would also explain why Mushroom and Koopa Castles look diffferent in every game; the remodleing must cost a fortune, but it's easy to make a fortune with all the coins floating in the air..