Yeah, its good, she's still viable at lvl 99 in PVP. You'll want to use her high agility, long range (to keep her safe), and people suggest to max her faith (97) so that she can land status debuffs on the short range bruisers while theyre on their way in.
Use frederika as debuffer, up her faith to 97. use concentrate to increse range, she have many debuff skill with long range attack, her agility make her move frist
In japan they use frederika to dispel High faith mage
you tend to get most of the units eventually in games like this, but if you don't get a good one immediately, you're stuck between developing characters you won't use long term, or not developing characters at all while you wait to land someone good. It doesn't ruin anything, it fixes the problems with the first week or 2 of gameplay.
Should i keep my acc with mediena sterne robb and full lb yshtola or my new rolled acc with mediena eolde ayaka frederika elaine?
I would stay with Full LB Yshtola, because you already have 2 other good URs, at one time or another Frederika will come for you.
Another point is that you will no longer be able to get Yshtola Shards, and, it is very traumatic to try to level more than 2 URs because you will not have any more rainbow resources, MR units are also powerful, think about it.