>>527609604
As long as you build your Yi Xuan around Fufu's buffs, she should be an improvement over Trigger.
Trigger is a multiplicative 35% damage increase, 23% against stunned enemies. She can also add 20% DMG increase if you have Qingyi's W-Engine.
This is what Ju Fufu has to beat.
Her CRIT DMG buff is much weaker than Trigger's multiplier increase, but during the stun (where you'll be doing most of your damage) it can be about as good if your build accounts for it properly.
Coupled with the Ultimate buff, the overall rotation damage should go up, and Fufu refunds 300 Decibels on every Ultimate use, and Yi Xuan has two Ultimates, so her Decibel one will effectively cost 2400 instead of 3000 on average.
Yi Xuan/Fufu/Trigger should be pretty good as well.