Is it acceptable to use a 15 A switch to control a 15 A duplex outlet on a branch circuit with a 20 A breaker?
Nope -- swap to a single receptacle

You cannot do this, as you could plug two 12A loads into the switched duplex receptacle and overload it. Oops!

In the Code, this is written up in NEC 404.14(F) (informational note omitted as it's irrelevant to the topic at hand):


  (F) Cord- and Plug-Connected Loads. Where a snap
  switch is used to control cord- and plug-connected equipment on a general-purpose branch circuit, each snap switch
  controlling receptacle outlets or cord connectors that are
  supplied by permanently connected cord pendants shall be
  rated at not less than the rating of the maximum permitted
  ampere rating or setting of the overcurrent device protecting the receptacles or cord connectors, as provided in
  210.21(B).
  
  Exception: Where a snap switch is used to control not
  more than one receptacle on a branch circuit. the switch
  shall be permitted to be rated at not less than the rating of
  the receptacle.


Switching to a single receptacle (one receptacle on the yoke vs a duplex receptacle i.e. two receptacles on a single yoke) for the holiday-lights receptacle puts you under the Exception though, which makes this all hunky-dory provided the switch is rated to the full 15A (1800W) that can be pulled from the receptacle.