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:
Select the material you've just created. In the Inspector:
Below, we've adjusted the Skin Smoothing effect to 100%:
Check this box to apply a smoothing effect to the whole scene.
Adjust the slider to make the smoothing effect stronger or more subtle.