(1) It is true that frost on your wing can thicken the boundry layer, and increase drag significantly (I'd stop short of dramatically).
(2) Frost won't have much effect upon air viscosity unless you're stationary. Kinematic Viscosity becomes a problem at high speeds and VERY low air temperatures. This is unrelated to frost.
(3) Any contaminant (including frost) on the upper surface of the wing has greater significance than an equal amount beneath.
Regards, and Happy New Year,
Old Smokey