I still find the idea of ⑨ amusing, but that's just me. I can see where it might get on some people's nerves though.
I think it became so widespread and popular because it turns the symbol ⑨, the number 9, into a symbol for both Cirno and stupidity, and also that it's a joke universal across language barriers. The average Touhou fans see a fan video, MAD, what have you, and may not be able to understand the memes/inside jokes within the lyrics or written out due to the language barrier, but ⑨ is instantly recognizable and doesn't need to be pointed out each time.
Amusingly enough, if it weren't for the ⑨ meme and all the jokes and bashing of Cirno, she probably wouldn't have gotten popular enough to have a ton of fanart/flashes/remixes/doujins about her, inclusion in 12.3, or Advent Cirno/other more serious and contradictory portrayals.
Honestly, I'm more tired of the Chuugoku/weakling/lazy Meiling meme because I think Meiling is sweet, and I feel like the meme was born in large part of Japanese prejudices/xenophobia against China.