XMB icons do not load correctly in 1.7.5 on Wii U. Instead, a font bitmap loads in place of the icons.
See image below.

I believe this is caused because the the assets themselves were updated in some form.
I had the same issue when I updated recently to some nightly near 1.7.5, and found out the icons were all missing.
You can fix this by simply re-downloading the assets, either from the online updater or manually.
I have this same issue, and downloading assets through "Online Updater" did not work either.
What bothers me more is that I placed roms in a folder:
SD root -> retroarch -> roms
I then scanned the directory, and none of the roms were placed into specific system directories, i.e., the Super Nintendo roms I placed in that directory to test didn't show up in a Super Nintendo specific directory in Retroarch.
This should work on the normal version of Retroarch, so I'm wondering if this is a bug with the recent nightly release for the Wii U version of Retroarch.
Uncompressing the downloaded assets worked for me.
Workaround:
Is the assets bundle too large for RA/WiiU to handle?
Kinda, the whole downloader is broken in 32bit, it always loops and
restarts, sometimes it works, sometimes it doesn't
on WiiU it's just... more broken.
On Tue, Nov 27, 2018 at 1:29 PM Rob Loach notifications@github.com wrote:
Is the assets bundle too large for RA/WiiU to handle?
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Can we download the assets.zip manually from somewhere?
The Download button at the top right. You can also git clone it to where your assets directory is set too...
rm -rf ~/.config/retroarch/assets
git clone ~/.config/retroarch/assets
Is there any Update?
I installed Retroarch the first time on WiiU and it is running but i get every time the message "assets are Missing"
Already tried to unzip the folder and put it into ./retroarch/assets or /retroarch/media/assets or the Zip into the media folder.
Every time the same, i tried it today with a fresh install with latest nightly (23.01.2019)
Put it into ./retroarch/media instead of those two. I tryed one of those and after doing it in media it worked for me
For anyone stuck on this still, I extracted the contents of the zip to ./retroarch/media/assets and changed the assets directory to that path within retroarch and it's working perfectly
Most helpful comment
I believe this is caused because the the assets themselves were updated in some form.
I had the same issue when I updated recently to some nightly near 1.7.5, and found out the icons were all missing.
You can fix this by simply re-downloading the assets, either from the online updater or manually.