Ok, I tested this enough now that I THINK this is the case, but not certain, curious what the observations you all have are… appears that biome type impacts the spawn rate of the hat mobs?
Caught onto it and started testing it and it seems that being in a wintery biome really increases the spawn odds. On my planet Stardrop Valley, on pure Ice Crack biome, I seemed to be running about 20-25% hat spawns. In another area, where ice and Snowy Redwoods merged, it was like maybe 30-40% if I had to guess there. Each of the times I returned to a saved spot there after testing other worlds, I warped into a pack of hat mobs.
To control, I tested other high level worlds, a few of my own, and the Rift exo. In areas that were temperate, spawn rate seemed comparable to Oortmas Land and Gyosha - pretty low, 5% or maybe lower. Could go awhile without seeing one.
IF this is the case here, well, know what I’m going to do… 
(Prob a T3 or 4 set up with easier snow and ice ones to run around)
A few festive Oortmas pics from the testing! 
