You can add a retouching effect to either the face or the whole scene, using a material in Spark AR Studio.
Next, create a material for the face mesh:
The material will now be listed as material0 in the Assets panel. Select material0 and then in the Inspector:
Below, we've adjusted the Skin Smoothing effect to 100%:
Select this box to apply a smoothing effect to the whole scene.
Adjust the slider to make the smoothing effect stronger or more subtle.