In tt, you need at least 2-3 of the following on any given champ(in order of importance); High damage, High bulk, Hard cc, Great escape. Karma has decent damage, but no natural bulk, not a whole lot of cc, and no real escape other than a slight boost. Most supports can get down the cc or the bulk, but don't have enough damage to be effective in a 3v3 fight.
The only "support" I'd reccomend is lulu. She has high damage with e+q, plus pix passive damage is really underestimated, hard cc with w and a slow with q, good speed boost with w for escape, and her kit can add bulk to any champ.
I've only actually tried her a few times, but it's been promising. In the one game I almost tied max damage to champs. Blackfire torch is a must. I threw on a malady for procs on autoattacks + as for pix passive.