I have NEVER EVER seen someone dressed up as a Nemo character.
And you are right. I've not seen a kid give a shit about finding Nemo in years. Same goes for The Incredibles, a Bugs Life, and Ratatouille.
They seem to be more cared about by older audiences. Kids have today have so much media today, with my daughter stuff is literally "here today, gone tomorrow." They only tend to remember something if it's a big, long running franchise with an onslaught of merchandise that's pumped out regularly.