While the possibility of a person faking this can always be there, as it will be until this is proven real, I really don't believe the idea that this is fake.
The Mario model replicates the Mario in the adventure mode pic we got like a day and a half before the leak occurred. It essentially matched perfectly, and compared to older models of Mario, it looks much more polished, which may simply be done for characters who are further along than others.
The dark lines and effects are consistent with basically every available picture from the Dojo or Facebook page.
The models are original as opposed to other pictures, so that would mean that within the amount of time between the Adventure mode picture and the leak, the leaker would have had to modify a Mario model, create a unique Palutena model, and then compare the two pictures to every single 3DS pic to make sure he/she didn't create any inconsistencies.
I highly doubt that scenario on so many levels.