I know it was just a comment made in passing, but I’ve seen it a lot and the whole Pokken Tournament outselling Street Fighter V thing is misleading. It outsold SFV at retail, which sounds somewhat impressive until you consider the fact that SFV has sold the majority of its copies between digital purchases on the PS4 and PC (neither of which figure into NPD sales numbers). Across all platforms and distribution methods SFV has sold 1.4 million units as of the end of March and I doubt Pokken was anywhere close to truly outselling it when this became a talking point (or now, when SFV is most likely still outpacing Pokken sales by a considerable margin).
However, Pokken did sell fairly well overall (a little under 500,000 retail units between North America and Japan by the end of April) and last week they hinted at another playable Pokemon being announced for the arcade version on July 14, so it looks like DLC may be likely. It's also doing well as a competitive game so far (4th highest number of entrants at EVO behind SFV, Smash 4, and Melee) and, as Greg mentioned, a data miner found files in a recent update that hinted at Scizor, Darkrai, and Empoleon possibly coming to the game in some capacity, so there's that adding to the case for DLC being on the way as well.