Hi @dark, thanks for posting your question!
If you haven’t already, you can post it as a feature request on feedback.vrchat.com.
To answer your question: VRChat currently doesn’t support streaming mipmaps. It’s something we might decide to add at some point, but some issues would need to be addressed first.
Most notably - some user avatars may lack mipmaps, have them disabled, or have a maximum priority. This would adversely affect mipmaps with streaming enabled: Once you run out of VRAM, they’d appear blurry and stuck at a lower mipmap. Users might run into this issue maliciously, or accidentally.
We hope this helps you understand why we currently don’t support mipmap streaming. Perhaps it’s something we can look into in the future!