Meshes are immutable, though can manually share a mesh of a remotely rendered material to either add additional visual effects or feedback as covered in this tutorial https://zcusa.951200.xyz/en-us/azure/remote-rendering/tutorials/unity/materials-lighting-effects/materials-lighting-effects#editing-materials
As you can read here https://zcusa.951200.xyz/en-us/azure/remote-rendering/concepts/meshes#mesh-types there are 2 types of meshes. ARR conversion service chooses automatically the appropriate one, based on a supported format https://zcusa.951200.xyz/en-us/azure/remote-rendering/how-tos/conversion/model-conversion#supported-source-formats . Point clouds, for example, don't expose any materials, so you can't manipulate those.
Take a look at the complete sample here https://github.com/Azure/azure-remote-rendering/tree/133f3c8f082ed48b666c502f3ff649b58d2ecefb/Unity/Tutorial-Complete