Does Machoke's "Daunting Pose" Ability prevent my opponent's Decidueye GX's "Feather Arrow" Ability from placing damage counters on my benched Pokemon?
Yes it does. Decidueye GX can still use the ability against one of your benched Pokemon, but placing any damage counters would be blocked.
Source: TPCi Rules Team (2017-06-01)

Relevant cards

Note: All applicable cards may not be listed below.
SM1_12
SM2_64
SMP_SM37
SMP_SM37