That shows how good the game is, the general rule is that the more porn parody something has relative to it's popularity, the shittier they tend to be. That or this game's population is 99% little kids.
Touhou is a prime example of this. It has a pretty strong cult following but the amount of porn it has is ENORMOUS, perhaps the most Rule34ed series in all of history, to the point where it pretty much can just be considered a hentai series now. And yes, outside the hentai, it sucks, big time.
Oh, and it's not just massively successful in Japan, it just might be the most successful game of all time. Tetris sold 170 million copies but we don't know how much each cartridge sold for. GTA5 sold 100 million copies, assuming 60 dollars each copy (which is generous), making 6 billion dollars in revenue. Monster Strike has made over 7 billions dollars and is MUCH cheaper to produce so it's profits is even more.
Base +6, flippernino -6, Shizuka Aoki -6, Megane13 -6