I don't think Karma is a mage.
I don't think Karma is a support.
I don't think Karma is a "mid"..
I think Karma is an AP Tanky DPS much in the same way Mordekaiser and Vlad are.
1. Tanky DPS generally lack hard CC... Karma lacks hard CC.
2. Tanky DPS generally have abilities which help them sustain in-lane... Karma has abilities which help her sustain.
3. Tanky DPS generally have SOME ability to burst targets down, while not having the kind of burst you would get out of a pure AP or pure AD champ. Karma is to Annie as Garen is to Tryndamere in terms of damage.
4. Tanky DPS generally require a large amount of farm to be fully effective. Karma needs farm.
5. Tanky DPS's job in a team fight is to:
-Be capable of harassing and lasting through the teamfight without needing to recall. Karma has heal, shield, and is capable of sustaining her mana after early game, while still having enough damage and range to give her team an edge if she harasses.
-Be capable of threatening enemy carries OUT of a teamfight, or peeling enemies OFF your carry when they get caught out of position. All 3 of Karma's abilities help her to do this. (AOE Damage+Heal, Haste/Slow in 1 spell, Spell Shield + AOE damage.)
-Make it difficult for enemies to focus them, whilst simultaneously encouraging them to focus them. Think about your average Tanky DPS here... They provide enough threat (Garen, Morde, Vlad, Sion, etc, etc, etc) that if you DO NOT respond to them, they will kill something... BUT if you DO focus them, they take enough damage to give their carries (Ranged AD/AP burst) time to clean up. So you either focus the Bruisers and die to the carry, or you focus the carry and get punished by the bruiser while you get kited. Again, Karma fulfills this role. She does significant damage and provides threat, while being difficult to kill.
It should be noted here that Karma does well solo-top, and she does well in 2v1 situations, AND she does well against other current tanky DPS top picks, who are mostly melee.
Karma benefits from Health Items, due to her passive... which means she is encouraged to build "tanky".
In my mind, here are some good Karma items:
Abyssal Scepter (AP, MR Reduction, MR)
Morellos Evil Tome (CDR, AP, Mana Regen)
Lich Bane (AP, MR, Mana, Passive)
Rabadons Deathcap (AP + Passive which improves her passive)
Rod of Ages (Health, Mana, AP)
Rylais Crystal Scepter (Health, AP, Passive)
Will of the Ancients (AP, Spell Vamp, Passive)
Hourglass (AP, Armor, Active)
Spirit Visage (Health, MR, CDR)
Sorc Shoes Or Merc Treads or Ionian Boots
Thornmail (Cheap armor if getting focused by AD)
Out of these, there are a few which truly shine on Karma. In no particular order:
Ionian Boots (Sorc shoes give flat MPen, which has bad Synergy with Abyssal Scepter which is MR Reduction)
Rod of Ages (Has everything Karma needs)
Deathcap (Makes your shield bombs REALLY hurt.)
Spirit Visage (Caps your CDR after Ionian boots, and gives HP and MR, as well as a boost to self healing)
Abyssal Scepter (Increases damage even more, and gives a nice chunk of MR)
Thornmail (for Armor/Defense) or
Rylais Crystal Scepter (For damage/Health)
Here's one example build:
I chose to go 9/0/21 for runes, and I picked up flat AP reds and quints, mana regen yellows, and CDR blues.. But you could just as easily go for 9/21/0 or 0/21/9, and go with Magic Pen reds and other rune options. CDR blues + 9/0/21 is just the easiest math for capping CDR.