Loss of quality in after effects 3D layer
Unless you need it make sure you are using the Classic 3D rendering mode and not the "Ray-Traced 3D" renderer. You can see and change the renderer in the top right corner of your comp view or just go into the composition settings menu through the top menu bar.

If you are in need of using the RayTraced 3D renderer click on the options menu and increase the quality from 3 (standard) to a higher value like 6 or 8 and see if that fixes your issue.
You may also change your AntiAliasing mdoe to cubic instead of box.

Also make sure your preview quality is not set to fast draft but to adaptive or final quality.