We don't know whether assist trophies can be added to the game as DLC.
It makes for rather bad marketing, really. Notice that all DLC we got has been related to gameplay - playable characters, costumes for Mii fighters and stages. An assist trophy is like saying "we couldn't let you play as this character, sorry." A Mii costume is easier to make and will sell better, to boot.
So, there is some logic to seeing Mii fighters as consolation prizes. We don't have proof one way or other about Mii costumes dis-confirming newcomers. People are hoping they don't because that would rule out K. Rool and Inkings who are both massively popular.