Do you have any in-progress photos showing the earlier stages of the shower construction? Such as the wall studs, backer board installation, waterproofing materials, shower floor construction, etc.

That's a lot of lippage. Do you have a sheet of the mosaic that you can examine to see if the tile has variation in thickness? I've seen some mosaics that do have that intentional variability.

Its almost so much that it looks like a purposeful choice. If the goal was to have this flat, well that is a fail. Talk to the installer immediately and let them know your concern. Ask why this is looking this way and if they can correct it. If they didnt get the wall 100% flat, then that would explain this. If they had to deal with stud edges that stuck out ect.

There's nothing that can be done at this point to make the lippage go away. And overhead lighting tends to accentuate any problems. Many types of tiles are warped during firing and people often aren't aware that this can be an issue until after it's installed. It's too bad that the tiler didn't check with you when he got started. Unfortunately, your options at this point are to live with it or to rip it out and start over. Sorry....

Poor quality import mosaics often have spacing and thickness issues. Id be more concerned about what’s under that for waterproofing.