It can’t be because the mochis were designed and created before Polandball. The oldest archived post involving the mochis is July 12, 2008, and that’s not even considering that some old posts from Hima’s yahoo blog are unrecoverable. 

As stated in my post about the mochis, fans made Hetalia emojis of the characters, and Hima made animated mochis from them. The mochis are a play on the word emoji (mochi + emotions = emochi ). Full post here [x]. 

This is confirmed in a note from volume 5.