When producing the final video with FakeApp (or similar app), you have the option of allowing the program to decide the settings of placing the face by turning Seamless to On.
If Seamless is turned On, the options for Blur Size and Kernel Size are being ignored. Seamless does a great job by reproducing the colors of the face in the original video. However, this output may be a little blurry.
You can manually set the placing settings by turning Seamless to Off and playing with the values of Blur size and Kernel Size. These settings produce a clearer image, but you might see some edges around the face.
Experiment with these settings to obtain the best output for your video.