I think her passive is actually really strong but is certainly feels like it isn't doing anything. I think instead I would prefer her passive to give a small bonus damage when charged instead of a free crit purely for the satisfaction of knowing your passive is pulling its weight even if it is essentially doing the same thing.
Though actually I think the best solution would be to change the particle effect on her passive shot so that the arrow looked bigger and more impressive then your non passive charged shots. Then you could see the correlation between shooting with your passive and their health bar getting chunked vs shooting without passive. Similar to how Caitlyn has fancier more menacing particles on her headshot proc.