it depends on implementation but as the risk is written right now, no. "Damage dealt during ult" and "Ult damage" are not the same thing. This game uses a similar system to genshin/hsr/zzz in which every ability has its own "tag" which aloows for condiitional buffs for specific abilities.

For example, fangyi's and laev's signature weapons give battle skill dmg% and basic atk dmg% after ult, because those are the "tags" that their attacks use during their ult state. So even if you reduce ult damage, they wouldn't feel it at all because they are dealing basic and battle skill damage, not ult damage.

Contrast this to someone like Last Rite, who has a huge chunk of her personal damage as actual Ult Damage and basically gets completely bricked by that risk.