Karma's an amazing champ, but she does have some problems.
Karma top lane can be good against some champs. She's good against Darius, who is a top pick right now. When Darius Grabs Karma, it puts her in a good position to Spirit bond a minion behind darius, landing both the slow and the damage portion. Mantra+Shield to deal damage damage, and absorb damage. Followed by a Heavenly Wave for additional burst. The slow from Spirit bond allows you to get out of his range Quickly after he pulls you in, reducing the damage he deals to you.
Darius is just 1 example. She can do well vs other melee bruisers such as garen. She's not great vs the hyper sustainers like Cho'gath, Yorick, and Vladamir.
Mid lane Karma is probably the Safest, and most effective. She can stay alive vs most Mids. Her harass is rather short ranged, and her sustain is rather mana intensive. If you feed her blue buffs, she can just spam all day, as her wave clearing is very strong. Her speed buff, along with her Shield makes her hard to gank as well.
Karma Doesn't have to build massive defenses to stay alive in mid like she does in top lane. Mid lane Karma can get away with Rushing Deathcap, and CDR. Giving her much stronger Shields/Heals. Which is the entire reason for Karma in the first place. Her burst damage is quite high as well.
Karma Bot lane is in a rather Iffy spot right now. Her Sustain isn't that great, and is very mana intensive, She only has a slow which doesn't counter agression, or save from ganks.
As of now Karma Bot lane is best played as a bursty Kill-lane support. Jayce/Karma, and Tristana/Karma have alot of burst potential, especially early game. This can easily get both your AD carry, and you fed. The only problem is that its a risky play. High CC supports like Blitz, and constant ganks from a jungler can easily backfire your Kill lane Karma Strategy.
Karma's Biggest Problem is her Lack of CC. No matter well you did in any of your lanes, if your team lacks sufficient CC you won't perform well in Teamfights. You have to build a team around Karma for her to reach her full potential. Here's a couple Team comps that will work with Karma
All of these Teams Capitalize on Karma Strengths, and compensate for her Weaknesses.
Bad Team Comp:
Each of these lanes can be strong on their own, but once teamfights come around, you won't have enough CC, and your Lack of initiate will almost guarantee you a loss.