I don't think Diona is the best healer in the game when the likes of Kokomi, Jean, and Bennett exist in the top spots. Diona is definitely a decent healer because of her good shielding and healing talents, but her burst makes it a bit complicated to get healed. Unlike Jean or Bennett, who use their burst in one place, Diona throws her burst toward the enemy, and you have to stand in that field to get healed. That can worsen the situation if you face enemies like Abyss heralds, lectors, or Geovishaps if you don't have a Hydro character to freeze them.