I dislike Shadow as well, but my point is people are going to waste their votes on some characters that probably have no chances.
I'm not sure we are going to get any third party characters after Oct 2015. It is a long process to inking some contracts between two different companies.
Filler characters or clones characters are waste of votes for few reasons. Other than returning characters, some people aren't that willing to buy clone DLC, and filler characters probably don't have good sales power behind it, like Dry Bone Koopalings, Birdo, etc. They need to pick characters that would make people want to buy it in order to move some sales for DLC.
Also there is 3DS limitation that could lead some characters being excluded or severely gimped their potential movesets.
I would be happy if I'm wrong.