Just watch this video and you'll see how "unpolished" her passive is. Sometimes she copies particles, sometimes she doesn't. I'm not complaining. It's definitely a cool and fun mechanic, but I'm just saying that it could have been better.
Maybe that's the point. Experienced players will be able to pick these little details and know that it is Neeko. I think it's intentional based on the art. Darius with the flower in his hair, Taller Nunu with the head tentacles, Ahri with a tail. All champions that are being replicated, but not perfectly. Something is off about them obviously Riot couldn't actually put that in the game due to MASSIVE art demands, but this simulates it using gameplay features.
