She's currently 10/6/10. This event is the first time the skill has been kinda useful, is it worth it at lvl 10?
Not really, since the crit down is quite situational and the NP dmg down won't really do much in later stages.
If you have resources to spare, or if she is your waifu, go ahead. Otherwise, I would recommend saving resources for someone else.
I'll say yes. Crit rate down is very useful and at 50% it's really good. NP down is situational but it's useful against the bosses like Abby or Amakusa whose NP removes buffs before it hits. The lower cooldown will helps too.
For general purpose, no. It's not worth it. As the other guy said, the skill is very situational. Nice to have when its needed, but you're not gonna need it all too often.
That said, if you like her and just want to see her at 10/10/10, don't let us stop you. I 10/10/10'd my Tamamo without any regrets.
