Your numbers were slowly declining, and with those ups and downs, I would not call that stable.
But the recent plummet probably has other causes. Could still be the opposite of going viral though.
Since you are on page 1/25 of top sellers for html5 nsfw, but only on page 15/137 of popular, something is going on. Page 5/137 might be reasonable. But 15? To be in top sellers, people need to interact with your game first.
My faforite theory now is, that the server your game is hosted on, did not report the access numbers properly, making the algorithm believe your game is less popular wich is a self fulfilling prophecy in the short run.