Honestly I think the time available to create this shows how impractical it would be if it were fake.
Providing there was 48 hours in between the Mario PotD image, and the leaked image, this would mean the leaker had to:
Create a Palutena model from scratch. Completely matching Pit's SSB4 artstyle, and consciously making her different enough to distinguish from other models. The fact that she shares similarities between Brawl and Uprising, while having new features, shows he would have to put A LOT of thought into this.
Next he would to have to have created the stage from scratch. This means, studying all the images available, and determining the exact perception in order to produce the angles viewed. In addition to this, he would to have noticed that a lot of background objects (Rocks, clouds...etc) are actually 2D fixed images, much like the food from previous smash games.
Third, with a new Mario image, he would have to either take an existing mario/ or create one from scratch, staying true to the most recent screenshot, and knowing to include appropriate design changes, that are consistent with a game that is further along in development.
Lastly, he would have to had studied outlines TO A SCIENCE. A common nitpick with these images, are that Palutena's outlines aren't 100% consistent, and dont cover exactly every area. However, many of the current 3DS screens display this exactly the same way. Luigis flutter is inconsistent, some weapons are outlined, and some aren't, pikachus tail is lighter, Pit's wings arent outlined.....and the list goes on.
If this does turn out to be a fake, which is a longshot, then this faker may possibly possess the best attention to detail abilities I've ever came across.