The list of acceptable conditions is foliage, heavy rain, falling snow, mist, and other natural phenomena.
Wiki even has a list of natual phenomena. None of which deal with “dim light”. In fact, most of that list deals with lighting effects, not darkening.
Looking at the plain English definition of Phenomena lists it as:
- something that is impressive or extraordinary
- a remarkable or exceptional person; prodigy; wonder
Dim light has not been remarkable or extraordinary for a very long time.
As a DM, I would also agree with the DM’s ruling: Dim light doesn’t meet the conditions to trigger Mask of the Wild