It depends on which context you are referring to. In the case of shielding and healing, Diona is definitely better than Kaeya because she has those abilities in-built into her kit. For sure, Kaeya does get a shield but only with extra constellations. If we talk about the Cryo application, then I must say I find Kaeya better than Diona because of his burst and low cooldown elemental skill.