There has yet to be a divine spirit class servant in the group with the host of their ability in-tact. They've repeatedly been given some rationale for why they aren't as powerful as their higher ascensions. The only exceptions to name were the elder gorgon sisters, who also admitted to being weak. Funny that.
As another example: Kiara, while not a divine spirit is every bit as dangerous as one, if not moreso but she's also severely depowered in comparison to Beast III. Again, the running theme here is that the Chaldeans are constantly denied from getting characters that are too OP. I don't see why Quetz would suddenly become the exception.
But even disregarding the status of Quetzalcoatl's saint graph, the problem remains that an entity that she could eliminate with ease would not have made any kind of difference in the battle against Tiamat. This contrasts how Gilgamesh was treating the bull of heaven like a trump card among every fighter he had and flipped when Ishtar lost it.
Don't get me wrong. Under any seen iteration, Lucha-neesan is obscenely strong compared to just about every servant and would likely tear Herc a new one, however the Bull of heaven instance is one hell of an outlier. That or Ishtarin botched the summon and it wasn't fully manifested for some reason. I haven't read any translations so I'm at a loss there.