By default in Unity 2022.3.22f1, under SDK Builder Validations, imported FBX models with a VRCAvatarDescriptor will always be rated Very Poor with exactly 2147483647 polygons. The fix is to enable Read/Write in the model's properties, under Model < Meshes.

In 2021, the same feedback post was marked Complete affecting Unity 2019: https://feedback.vrchat.com/open-beta/p/u2019-vrcsdk3-avatar-202107151344-newly-imported-meshes-have-readwrite-enabled-t