Are there any wii u rom hacks?

First no such thing as wii u rom but there is afaik some mods like translations and censor removing kinda thing for some games.
Check out geckiine. I've never used it but it's one tool used to modify games on the wii u.

 

There's some Wii U games on gamebanana but I don't think any of them have more than character and music mods

 

there are wii u roms technically. the ds game for cbhc is in a file named rom.zip.

there are some additions. I know Joonie of team rebug from the ps3 and switch scenes told me he made some mario kart hacks. I've never tried a "rom hack", but be careful with them.

 

CTGP-U is good if you have MK8

 

fyi, Joonie is from team rebug. he likes the wii u pretty well. that could be why he helped develop a modchip for the switch.