RetroArch - NES - Mesen
The texture_packs/RetroArch-Mesen/ represents /retroarch/system/HdPacks/ folder.

Note:
Some texture packs could made for a specific version or region of a game. Make sure you have the right game and textures for it.

Enable Texture Packs in the Mesen core for certain games
From the RetroArch Quick Menu

- Go to Core Options -> Enable HD Packs set to On
- Then save the current configuration for that game under Core Options -> Manage Core Options -> Save Game Options

How do I add texture packs that can be used by the Mesen Core?
NOTE: On the Steam Deck this could be easier to do in Desktop Mode.

- All texture packs go into the texture_packs/RetroArch-Mesen/ folder.
- The texture pack have to be extracted from .zip or other compressed format into a folder.
- The name of the folder must be the same name as that of the rom file the texture pack is for.
Example:

You want apply a texture pack to a game called ScaryCastle USA.nes and you have downloaded the ScaryCastle TexturePACK HD.zip.

- Extract the ScaryCastle TexturePACK HD.zip
- Move the extracted ScaryCastle TexturePACK HD folder into texture_packs/RetroArch-Mesen/
- Rename the ScaryCastle TexturePACK HD folder so it has the same name as the rom ScaryCastle USA without the .nes file extension.
- Start up RetroDECK and select the ScaryCastle nes games.
NOTE: On the Steam Deck go into GameMode first, then launch RetroDECK.

Related wiki article can be found here:
https://retrodeck.readthedocs.io/en/latest/wiki_emulator_guides/retroarch/retroarch-mesen-texture-pack/

The RetroDECK Team