If you insert a high-resolution image into the surface and it doesn’t quite look right, it could be because the surface isn’t scaled to the largest part of the shot.
Mocha assumes that any adjustment to the surface means you want the scale at that frame to be 100%. If you adjust the surface when it is very small in the shot, the scale will only increase as the surface gets larger, resulting in the pixels being rescaled to match.
Solution: Try adjusting the surface on the frame where the surface looks the largest, even if it is just moving a surface corner then putting it right back where it was. This will reset the surface to be 100% scale at the current frame.