How common is it for people to try to decorate Trevenant up as a Christmas tree and how well does that tend to go? Just saw a drunk guy getting chased by a Trevenant with ornaments on it.

It depends on the region, anonymous. In Kalos, it’s rather common, especially if the trevenant is tame. Tame trevenant tend to be raised from phantump if not exposed to Kalosean culture at an early age, so the act of decorating them either invokes memories of their past lives or allows them to participate in a tradition they’ve seen their humans prepare for months in advance. Christmas in general is rather big in that region, so even the wild pokémon of Kalos are familiar with its imagery (which in turn makes them a little more accepting of it).

On the other hand, in regions where it’s less common to decorate pokémon or celebrate Christmas to begin with (such as in the cases of Alola and Hoenn, respectively), it’s not that common, no, but it’s a practice brought in by foreigners. Such as your neighbor, perhaps.