Update 04.04.2019: It only took Capcom 6 months to address this blatant issue but it seems as of build 167353 they have finally fixed the texture quality setting and thereby have remedied the low resolution problem. I recommend wiping your nativePC folder to clear out any textures touched by this workaround. You do not need to use the High Resolution texture pack by the way, the issue is fixed for the "High" setting as well (I haven't verified others for obvious reasons).

Update 31.10.2018: Capcom has partially adressed the texture loading issue in today's title update (Kulve Taroth). Several reports indicate the issue still persists outside of the Astera hub. Hence why I will continue to update this workaround until it's fixed completly.

This is definitely not a proper fix for the problem and it might introduce other visual curiosities even if I haven't noticed any.
Currently adresses player, armor, NPCs, Palico and weapons.

Tested and working with game build 154766.
Use at your own risk! Play in offline mode if you fear a ban or whatever.

Note:
If you have installed texture mods for the above categories, rename your nativePC folder before running this. Then, after the steps below are completed, copy back the contents of that renamed folder to nativePC. 
If you have to overwrite texture files in the process, those will  likely suffer from the low resolution issue again.
I can't and won't guarantee compatibility with mods touching NPC/weapon/player/armor stuff.

How-to (First-time install & update):
1. Move WorldDecompress.exe to your game install directory (i.e. D:\Steam\steamapps\common\Monster Hunter World)
2. Unpack all main WorkaroundData*.rar from the download section to your game install directory so that the *.cdi and *.cdp files are next to MonsterHunterWorld.exe and WorldDecompress.exe
3. Run WorldDecompress.exe and wait for a bit
(4. You can now delete everything called WorldDecompress_* from your game install dir)

Tip: You can install WorkaroundData independently from each other. No need to reinstall everything if a new one gets added. Just put the .exe in the directory as well as the *.cdi and *.cdp of the new WorkaroundData you want to install and run.

To get rid of the mod, just remove the nativePC folder. Or, if you have other mods installed, remove the files of this mod to keep the other mods intact.
If you downloaded this mod from anywhere but NexusMods, be wary of malicious modifications.

Enjoy!

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Technical background: The game loads lower resolution mip-maps for some textures than it is obviously supposed to do under certain circumstances. This workaround forces the game to use the full resolution by editing the original texture file. Obviously, low resolution mip-maps exist for a reason, so visual artifacts may occur. Capcom unfortunately hasn't acknowledged this issue as of 23rd of August 2018 so I decided to release this workaround.