Ok

She's good, but I've ended up mainly using her in Hydra (Normal/Hard) since Magnarr is a better Arena spirit nuker. Would want her in Arena to counter specific shield heavy comps. 

Thanks, I have Magnarr but I hadn't really considered Hydra

Though with Bolster I presume Yannica's value goes up, even if half of that A3 is negated. 

What are the best artifacts to equip on her? Stats? Sub stats? Specifically 

For Arena, typical nuker build, 100% crit rate, 170+ SPD and as much ATK and Crit Damage as you can get. Ideally, with CD gloves and in Savage set.

For Hydra, all of the above, plus around 250 ACC for Normal and Hard. If possible, give her a bit of survivability or pair her with good support, bc she is squishy.

Is her value in Hydra enough to use clan keys to get her?  I have been using the keys for energy.  

Do you mean Clan Gold?

You should be buying her fragments every week, yes. As far as I can remember you don't have a spirit nuker yet. She can do that for you.

The energy is pretty bad value, but if it's necessary, it's necessary. The fragments and accessories should be coming first, though.