Nope. Nuh-uh. Not happening. If you're a fan of Super Street Fighter IV Arcade Edition or King of Fighters XIII on Steam, the Steam Controller has nothing to offer you. Fighting games, by the very nature of their special attacks, super moves, and complex combos, require you to press multiple buttons while holding a direction. On the Steam Controller, that seems all but impossible, because your right hand is hamstrung by the giant circular touchpad it has to jump over anytime it wants to press a button. And the buttons located by the left touchpad might as well not exist--you'll never be able to press them during a match.
Is this a huge loss? Not really, in the grand scheme of things--PC fighting games have always been a niche, and those that do play tend to prefer an arcade stick with a console anyway. Still, it's bizarre to think that an entire genre with a decent presence on Steam will be wholly incompatible (in theory) with the Steam Controller itself.