I just really don't think it was neccessary to break the rule: Materials are used to craft items. (And indirectly also: Tokens are used to trade for items.)
You added a merchant that trades a material for a costume helmet. Why. Why not just follow the existing patterns and make it something that you can craft in the Mysterious Alchemy Machine?
It bothers me because it was a design rule that made sense. Rules like these are there so that it's easier to understand the game, and to make it more coherent.
On that note, you also didn't even bother to replace the Token icon when breaking the rule by repurposing the token trading code to implement material trading npcs. (The npc also looks awful, but that's a different issue.)
I don't hate breaking rules per se. I appreciate that you're making costumes that look different. I don't hate the qol updates. I just really don't see why you'd want to break this rule.
As with Apocrea's Obsidian Edge moulded into a costume shield as Obsidian Obstacle, I wager the design purpose of these being restricted to event NPCs is to get people to show up for them during the timespan of the event.
It'd have been nice if it (and Obstacle) were at least a real item with stats, even if it were gimped (tortoguns, Grand Tortoise) or underpowered with the occasional niche they're okay at (the other tortoshields).