Complaining and just understanding something from another's view point aren't the same thing.
However if this item was only available from the end boss in mythic level only raids, I can imagine there would be people complaining about that too, and the same goes for if it was rewarded purely from winning say 5 arena games or similar.
My point being, that for a limited time holiday event, I personally do agree that I find it to be rather silly to put it behind a mythic only (non queuable content) dungeon boss, nothing stopping them from putting it on a normal or heroic mode dungeon boss, so that the majority are more likely to be able to get it.
After all it's just a fun holiday item on a limited time frame.
Myself personally, like I said, I don't need it, and even if I did, I don't do mythics or even dungeons in general so I just wouldn't bother anyway. However I do see it from others PoV.
There is no need to really get defensive about it at all, I mean putting it onto a normal or Heroic dungeon boss to give even more players a chance at it, is that going to affect you negatively?
Just my own thoughts on the matter