The title ID of every Sega Master System game starts with L, and every Sega Master System title the console puts it on 4:3 automatically, the title ID of the Homebrew Channel it's "LULZ", the Wii U thinks that it's a Sega game and puts it on 4:3
To fix this you can get a Forwarder for the Homebrew Channel, a Forwarder of the Loader that you are using or a Homebrew app that puts HBC on 16:9 once you enter to the HBC and load the app, I don't remember the name of it tho.
You can install any Forwarder or wad in general with "Yet Another Wad Manager Mod" (YAWMM)
Only install a wad when it's compatible with vWii, obviously, if not you will brick the vWii.