Unctuous:used to describe someone that is meant to seem friendly but that is unpleasant because it is obviously not sincere. (By Merriam Webster) So unctuous smile seems appropriate in this case to me.

Sardonic:grimly mocking or cynical."stark attempted a sardonic smile". So I think"sardonic" smile is apt in this case because it also highlights the disapproving factor , if you follow other definitions.(from free dictionary: characterised by a scornful derision , bitter irony )