As I always say, no character is bad in the game, and comparing character heavily depends on the context. If you are talking about off-field Hydro application, Kokomi outclasses Barbara in every aspect. On the other hand, if you are talking about a healing loop that follows your active character, then Barbara wins. Depending on your requirements and the type of enemies you face, you can use Kokomi or Barbara in any way. However, Kokomi does perform better than Barbara when it comes to Hydro application, healing, and normal attacks. So, if that's your requirement, then, by all means, go for Kokomi. Still, Barbara can be used in another team in the abyss.