She's sitting on a 45% win rate (That's abysmal) according to champion.gg, with a very low pickrate as well. I've played her for a bit and tried to get her to work, but she's legitimately an awful DPS.
If you want to buff her, I have a suggestion for a QoL buff to Kalista: Give her Rend a short, uninterruptible cast time that triggers martial Poise. I noticed that lategame, when my passive is all that's keeping me out of the hands of melee carries, Rend is an effective way to commit suicide because it breaks the cadence of when you have to click to activate poise.
It should get the same casting time as a basic attack and trigger martial poise to allow her to use Rend while chaining together lunges.