Creating a Custom Title Screen
Tools Required: Miyamoto Level Editor Switch Toolbox
Of course, make sure you have a rom obtained from your Wii U console. A guide to dumping games can be found here.
Importing a custom logo
First you're going to find the layout.szs file. (This contains the asset used for the the title screen logo) So go to this directory in your NSMBU rom: content/common/layout..