Screen mirroring is currently limited to resolutions up to 1080p. Computers that use a much higher desktop resolution, such as 4k, will have the mirrored image converted into 1080p in order to match compatibility with the receiving device. This conversion process may naturally lead to some image degradation. 

In this case, using a 1080p resolution on the sending computer will eliminate the need to scale the mirrored screen and result in a sharper mirrored image.