The source of this bit of information does seem to be pretty watertight being the eShop itself, but until something official is announced, it is always prudent to take things with a pinch of salt. Wario, made even more famous via his extremely zany WarioWare series, has appeared in the Super Smash Bros. series before, appearing on the Nintendo Wii in Super Smash Bros. Brawl. As for his purported upcoming appearance in Super Smash Bros. 3DS/Wii U, it looks as though both of his classic and biker outfits will be made available.
We do look forward to an official announcement in the near future though, so hopefully something along such lines will appear before Super Smash Bros. arrives on the Nintendo 3DS later this 3rd of October 3, while Wii U owners will have to wait until the holiday period.