Karma is one of the best and underrated champs in this game.
However, she is stuck in the role of AP mid; and has no hard CC.
As such, the team has to compensate around her and pick up CC in other lanes. Further, Karma's late game falls off slightly in terms of damage but increases in terms of support. Making her rather unique, but bad for being the focus of solo mid farm/exp without choice teammates.
I can out-burst LeBlancs to all hell (shield nuke + heal nuke and DFG dropping her to about 20% while my damage mitigation in my offense cuts her damage output to a pitiful amount in comparison). However, LeBlanc still lives and my mantra is out. Karma likes committed fights. She isn't the best at trading cool downs; and I have to wait longer for round 2 than she does. Late-game you need coordinated plays to make good use of your mantra.
Riven or Jax top would be ideal; melee hyper-carries that can get into the center of the fight and make good use of your shield-nuke and leash. They also bring AoE stuns to compensate for your lack of CC.
Lulu, Janna, Alistar, and Sona all make good support picks for their shield/heals/CC to synergize with Karma's kit and pick up the slack in CC.
And the jungler must also bring CC, someone like Amumu would be pretty ideal. Nautilus/Alistar/Maokai/Shen also very fine picks. But someone like Nocturn wouldn't go well. Sure you can keep him alive and what-not, but with only a single target fear and his late game fall off in tankiness... won't be too pretty late game.
A farmed Karma is also very good at keeping Vayne alive.
There are many ways to break out the true potential of Karma and be a true terror. However, she is a very specialized pick.
Akali is an assassin... You simply pick her if there's a lane she can survive and squishy carries she can kill.
Much simpler for you to learn Akali and opportunities to use such skills would be more often available.
However, since Karma is underplayed, you may find it rewarding to master an underdog and make your victories ever the sweeter.