Personally not my thing, and to be honest, of all 20 friend who I play Smash with and geek out over Nintendo with, we all have the same opinion on Splatoon.
Boring lame game that's fun but extremely basic and we wish Nintendo wouldn't focus so much on.
It's just weird to me seeing the love Splatoon gets online and in Japan when I literally never met anyone in real life who likes Nintendo that is actively excited with the franchise.
I'm one of those old guard Nintendo gamers who don't get Splatoon. From what I can see, Splatoon's fanbase are in their teens/20s, so kids who grew up on Wii/DS and GameCube. Different generation.
I don't begrudge it at all, bigger tent is always good. And there's no evidence Nintendo's focus on splatoon is affecting anything. The game's lead designer is among the crop of young developers at Nintendo. Aonuma, or Koizumi isn't being asked to contribute and Miyamoto is virtually retired from game development at this point.