I can probably imagine why they wouldn't/haven't added this sort of thing in yet. It'd be hard to make it fit along with the rest of the trinkets. There's other weapons that are set to their own few damage types and it'd be awkward to have certain weapons deal certain damage types (and also may be balancing such as well, but I imagine making things fit and make sense is still a small part of it) Although a "piercing trinket" doesn't make all that much sense if I'm going along with making thing's "fit", so having no normal damage resistant trinket might just be for balance along side the normal damage only armor sets and the possibility of getting normal max UV on them, but why doesn't a Stun resistance trinket exist then?
I am unsure of how stun resistance works, but there's armor sets out there that'll give a full bar/resistance and we're able to use our Sprite Perk that gives stun resistance, so I'm not entirely sure why there couldn't be a stun resistant trinket other than being unable to make it look decent or fit along with the rest of Spiral Knight's content.

I love the new Starlit armor sets and I have the Hunting set, and one of the main reasons I hate pulling it out but I end up using anyways (I have my own good UVs slapped on) is that this set for some reason has a whole lot of negative stun resistance, unlike the rest of the wolver line sets so it makes going for the Starlit sets not all that worth it. There are only very few slumber levels and that they may have slapped this negative resistances on the sets, but not many people are going to make these sets other than for their looks (which both look pretty great) or getting that sword damage and sleep resistance for LD. This sleep resistance in the arcade isn't all that great, though. You may be able to survive longer walking through sleep clouds from Sloombargos, but if you get slapped by an rng needle spin attack or a melee from one of these slumber enemies, you're still bound to get chunks of health taken from you, either by getting slapped with a hit that sleeps you and getting hit again taking another chunk of health, or while you're slept they don't deal enough damage to wake you from your slumber so you get hit another time wiping you down to 5% or death, so having this negative stun resistance is a real nuisance, with the Lumber's slamming down, or Colonies spinning around giving you a slap and making you insanely vulnerable to getting hit, especially by the Slooms and the Sloombargo, almost guaranteeing death if you don't have remedy or proper trinkets.
Players might as well craft up the Slime armor sets to get that stun and sleep resistances, overpowering the Starlit sets. Even though they may have weapon buffs, the rest of the Wolver line and Bomber sets (excluding mad bomber) all have status resistances, weapon buffs, and no negative resistances.
It'd be nice to see Stun resistance trinkets, but Brink's shop is already pretty even and filled so I'm not entirely sure where these would be implemented, or what they could even come up with to make a fitting texture for these trinkets. Possibly being a Krogmo crafting recipe, since there is no status/damage resistance trinket being sold/crafted with Krogmo materials, so this may be a good spot to slap it in.
Or I'd like to see a buff to the starlit sets, but I don't see something like that occurring anytime soon.
Or possibly a change to how sleep resistance works, since it only seems to decrease the amount of time you're slept and I'm unsure about enemy physical infliction of status effects and how a status resistance affects that at all. The change possibly being less damage needed to be awoken from enemies to reduce the chances of needing to be hit multiple times to be... awoken? I'm not entirely sure how to say this properly if this is incorrect.

I typed this over a period of time and put more ideas in between paragraphs and words so I'm not fully confident this post is coherent or understandable.