Server-side Fabric mod that allows players equip items as hats.
To equip an allowed item, simply put it in the helmet slot - no commands necessary!
The mod is very customizable, making it easy to tune it to your preference.

Requires Fabric API

Features

By default, mod allows players to equip most items that have special rendering rules in vanilla Minecraft:

Banners
Fence gates
Amethyst clusters and buds
Beds
Feather
End rod
Lightning Rod
Spyglass
Lead
Bone
Cod
Conduit

And some other ones I thought looked cool\funny:

Glass blocks (including tinted and stained glass)
Azalea bushes
Scaffolding
Big dripleaf
Slime and honey blocks
Composter

You can remove/add any items to this list. You can also make it so any item in the game can be used as a hat!

These items can also be equipped to players with a dispenser and can be enchanted with curse enchantments on an anvil (for when you want to test your friendships).

Shift-clicking these items will not equip them - that way players won't accidentally end up with their building blocks on their heads.

You can tweak all of those features using a /hats command, or a configuration file. You can read more here:
https://github.com/WerDei/Server-Hats/wiki