YW is only becoming more popular than Pokemon among elementary children, the target demographic of the entire series.
The game was initially not popular at all. What has made YW the sensation it is atm, is the anime series, which again is directed at young children.
While JHS kids and up still have various liking towards the IP, Pokemon is most definitely still more popular and will remain so for a good time.
On another note, I do not see the game doing well outside of Japan. And that is because of its battle system. This game is no Pokemon. Its no Ni No Kuni. It plays like a mobile game.
That is to say, in combat you have no control over what actions your creatures do. Nor can you tell them who to attack. Everything is automatically done.
I am not sure if that will appeal to a large crowd.
That said, when I finally pick up a 3DS I will pick up YW, but only because I want it for Japanese practice, and because its cute.
Will be interesting to see how the IP is leveraged in the West. Nintendo should jump on a WiiU version though... the console could seriously use the help in Japan. But whether Level 5 cares enough to do that is another question.