Well, they did import Min Min's model too, didn't they? Though they cleaned it up a bit. Thankfully, ARMS and Smash have similar aesthetics.
But on the other hand, creating a model can't be that hard. Every wave we get a handful of new models in the Miis. And some of the DLC characters even debuted new models just as background characters, like Banjo and, afaik, Joker.
Plus, all things considered, shifting Minecraft Steve to Smash Steve would only entail fairly minimal alterations, when compared to some of the other characters they've modelled. And I bet you there is some small difference somewhere - indicating some changes were made.
It does seem like this character was quite work and resource intensive, I just don't think the actual character model is the area they'd choose to skimp on. It's one of the most important aspects. It does seem like it was a deliberate artistic choice to me, just one I wish they'd not made.