If you play her well she is also a pretty good counter-pick to many top-lane match ups. Take flash-ignite, use her q to wear down the enemy. Once they hit below 1/2 health wait til you have 2 mantra charges and go in for the kill w/ mantra'd e, q, mantra'd e, ignite. When they come back to lane, keep the pressure on so they can't catch up. Also shielding minions can deny last hits. Build her as a tanky mage (similar to the way one would build morgana). Her early and mid-game burst is amazing and unexpected, but since her role changes to more support-y and sustained damage late game it's wise to only use her on a team that has other significant sources of burst magic damage.
One important note: Karma's support presence is very strong, but it comes off in a way that tends to inspire overconfidence in extended skirmishes. If you start to get nervous, that's a sign it's time to disengage. Your teammates will feel invincible and they will stay in a skirmish as long as you are present, but they don't know how to gauge your resource management, so it's up to you to let your team know it's time to leave. I can't tell you how many times myself and 2 others have got 2-3 kills over the course of a few minutes of skirmishing, only to give up an eventual triple kill.