She no need more defense on this spell, give her an offensive component on shield non mantra and she will be a very strong mage and support but since her offensive power is stuck on two spells (Q and W) it feels bad. Riot here are two options to make karma feel really good when going mid or support.
Option A:
Give Karma's shielded allies (including her) a small ap ratio on there next auto attack or attacks for the duration of the shield. This is a simple fix and will give her more poke in bot lane and have great synergy with caitlyn, ezreal, kog maw while also giving her more reliable damage from mid lane and top and opens up damage options away from just Q or W. This also really helps jungle karma. Give it a small ap ratio of .2. And an appropriate nerf if needed is to lower the ap ratio on W, which currently as a .9 ap ratio.
Option B:
Bring back her shield bomb to please the masses but adjust it accordingly to make it fit her kit. Make it have a smaller radius but give it a small aoe knock back. Making it a mini janna ulti with damage instead of heal. The knockback is really to make it worth while to mantra shield while not have levels in E. As mantra W currently feels good at all levels this with the percent missing hp regen.