I never seen Karma in Solo/Duo queue. I consider her a Very Good support. She got nice CC and nice Heal, Buffs, but I don't understand why people are not playing her at all.
Karma is just not very suitable support, she needs farm and items to be effective. That's why most people who play her play AP carry karma mid or top.
Her early game is huge with shield and healing wave(which heals and deals dmg) and this is the build i found most effective: Start with boots or dorans ring(if you dont expect early ganks) and max shield first(dont take point in your chain skill which is E) and max Q 2nd and take E skill only when you must(i think lv15). Build your WotA(get revolver at 1st buy) and couple of dorans rings. Proceed to rabadons deathcap and rylais crystal scepter. For boots buy ionian boots of lucidity or mercury treads(since that +20 Mpen in not very needed). Other items are situational and are something like void staff, abyssal scepter etc.
For laning i suggest walking to an enemy and activating mantra, then your shield followed by q and repeat it each time you get 2nd mantra charge. If you are unable to come close to an enemy activate mantra and shield a minion near the enemy champion. Teamfighting is basically the same thing but you may want to heal your allies a few times in teamfight(well placed heavenly wave can heal all your team).
Hope this helps!
You can try this guide for reference http://www.mobafire.com/league-of-legends/build/the-comprehensive-karma-72191 This guide seems to be liked by many people. And for your previous question I've heard something about Karma not actually being that helpful as a support and would be better as an AP. I can't really give a judgement on this since I do not play this champion. I hope this helped you some.
Karma is my favorite champion in the game, so I'm glad you asked this!
There are currently a lot of cons to karma that keep people from playing her. Also, some people just dont understand her.
Karma has a very low range and an incredibly low movement speed. Her mantra only has two charges and they take awhile to replenish. When your mantra runs out in team fights, it really hinders her playing ability.
Karma just doesn't live up to the other supports or ap mids. She is a wonderful hybrid, but it's just difficult to find a time in tourney games to use her.
Actually that's a good question because I personally think that karma is a very strong support. However, most support players really like using easier to play supports (ie. Sona or Soraka). I guess the number one reason I've heard people say is because she doesn't have that much of a reliable heal considering she has to activate her ultimate in order to use it. Honestly I have no idea why she isn't played that much because she is a very good support - just look at her kit to prove it:
Q: AOE heal that also does a good amount of dmg in case of those engages bot
W: Either slow enemy or speed up ally with the potential to slow more than 1 if correct positioning
E: Huge shield with massive AP scaling that again does good dmg if used with mantra to all nearby targers
R: It's free, lets her get 6 points in other abilities and makes all of her abilities a lot stronger so really she's very good. But since she's not played in tournaments because no one is used to her, and since she's not used in tournaments, people just assume she's not good
It's because she is in the middle of a carry and a support, she can nuke people, and support quite well. but since she isn't one specific role, you can pick any other champion to fill either of those roles and it would be a better choice than her. They are working on her at the moment though, so be prepared to see her a lot more often.