She is good in soloQ (may god nagakabouros have mercy on SoloQ's soul) because she is a more well rounded mix between Lux and Janna support. Or put differently; she can deal the damage Janna wishes she had and has the utility Lux players try to convince you she has, when they lock her in to play "support", but buy mejais, rabadons, dont place a single ward and steal all kills.
The good thing about her is that you can pick her and then be useful in many lanes, top, mid, support and even jungle if you mastered her (it's still cheesy tho). You can very easily make up for most of your team's shortcomings.
That being said, if your ADC knows what he is doing and you are reaching plat+ levels, you are going to be more effective with other supports. People learn to avoid your poke in lane, junglers will abuse the fact that you push the lane and that you have very soft disengage. And lategame the catch potential and hard disengage of other champions just becomes more useful than Karma's damage and soft... everything.
Karma's root and slows are nice, but it's nothing compared to the disengage or crazy stuff some Thresh, Braum or Bard mains can do.
Also Karma's damage looks bleak compared to the kind of damage a good morgana root or thresh hook allows your ADC to do.
I would say that Karma is a very good support from bronze to gold and a decent one in plat. If you wanna main support in diamond, you will probably find yourself playing her less and less.