Trinkets don’t do anything: literally nothing… Aside from adding a little more flair to your weapons, they have no other use.
Weapon trinkets dangle off the side of the gun you have it equipped to, which is enough for some. It’s certainly enough for me.
How to Equip Weapon Trinkets
Equipping a trinket isn’t all that different from equipping a skin to your weapon since both have to be done from your inventory. (You can equip a skin just by pressing the Inspect button/key when highlighting a weapon in your inventory. Easy peasy.)
Popping a trinket on your guns isn’t much different.
Open your inventory and shuffle your way over to the weapon you want to slap a trinket onto, then press left on your D-pad or what have you — I’m sure this is just done with the arrow keys or something with KBM, though I can’t comment — to select the little empty box to the left of the weapon you want to attach a trinket to. Choose one and you’re all done.
Equipping a trinket does not consume it, meaning you can equip one trinket across multiple weapons as you play the game (that’s right, you can slap that diamond ponytail on every weapon if you want). No harm in making use of them, if you want to give your play a bit more flair.
Weapon trinkets may not mean much to some Borderlands 3 players, but any bit of customization you can see pretty much all the time is A-OK by me.