Is Kiyohime Berseker good?

Well... I think she is better than Darius
Especially when her third skill is maxed

She's good also she has an AOE Np to help farming

Yes, she's good and really good when fighting multiple enemies or servant with huge HP.
She's really valuable when clearing event faster and If you have Kaleidoscope or NP battery CEs, she does work like a budget Raikou.
Having multiple Zerker can help when dealing with enemies with so much classes, one can act as your back up DPS.

Asterios for that massive debuff, Darius for some durability/harder content, Spartacus for 3 turn farm/NP hit pretty hard like Kiyohime, Lu Bu for one-shot enemy Bossess, Eric for budget Heracles, Caligula/Dr.Henry Zekyll for Crit zerker.

Also if your skill is good enough as a master/her pilot, you can even bring her to kill single boss with large HP.
Good servant + skilled master = GGWP.

Depends really. For farming, she is more than sufficient as long as you have a way to get her NP off quickly. For actual bosses and stuff like Barbatos, not really. She does some good damage with her NP, but she has a hard time charging it because of her deck and she has no real way to survive for long unless you build a party specifically around her weaknesses.

She is my main farmer Zerker (specially on hands), and yes, she's pretty good.

While Darius does have a more powerful NP (only low star AOE Zerc with interlewd strenghtening his NP...), her Strenghtening Quest Skill, Flame Colored Kiss, makes up for it.

You can even use her as a pseudo-boss killer, thanks to Stalking reducing some1 defense for 3 turns, but be careful with the attack up demerit.

If you Have MLB Halloween Princess or Golden Sumo, it stacks multiplicatively with her Buster Up, lus the 50% NP gauge, for friendly Waver/Zhuge to full charge and clear almost any mobs.

Sadly she has no hard survivability (Gutts, Evade, Invulnarability), and Morph C is almost irrelevant, so, don't expect her to endure a lot of time in the frontline.

Also, she has a triple B deck, even with a sorta decent NP gain, dont expect her to use her NP more than once a node, unless you do things like double-Waver+Kalidoscope.

She is better than a lot of 4* Zercs, like TamamoCat and ST Beowulf.