Karma doesn't really have a place in the current meta. I've heard of her being played top, but I don't know much about top as it's my weakest lane, so I can't really speak to that, and it doesn't really make much sense to me, since she's not tanky at all. As a mid laner, she has a short range, no gap closer(like an assassin mid would have), and a rather underwhelming burst. As a support, she provides minimal CC(just that tether thing, and to make it even remotely effective you also have to burn a mantra charge), and she requires alot more AP to function effectively then most supports can afford to build.
Basically, her abilities all fill a support role, moreso then anything else, but to make them even remotely effective, she can't build like a support. I like her, I own her, and I even own a skin for her, but I can't imagine a situation where I would actually find her preferable to another champion.