Is worst IV Witch Celica worth keeping?

If I were you, I would wait. Try and pull for a better IV if you can. Also, people are saying that she will most likely get demoted so if that happens, you won't lose much if you fodder her.

Res is a superboon, and you could use that to make her tank Myrrh and Grima Robin much better. Give her Close Def A slot and Quick Riposte seal, and ta-da! She's still amazing!