random textures for mobs could be great, however I believe in variants that don't change every time I relog, and while using the uuid might be a good option if things are added or removed it will reset, I propose the ability like names and teams to set a texture based on an entity tag, this way modpack / datapack creators can make custom variants with unique textures without implementing a brand new mob.

Does another mod also do this?
not that I can recall

Does OptiFine do this?
not since the last time I checked

As thanos covered above, entity tag information is server side only and is not sent to clients. So there is nothing etf can do.

This problem extends to some other nbt data as well.

While this would be cool, I believe it would require changes to the server as the server doesn't send tag data to the clients, so this is likely beyond the scope of ETF. Potentially a different server side mod could send tag data to the client, and enable this functionality, but I'm not sure of any mods that can do this at the moment.