As long as it’s a mod and not a full game, it should be fine. Typically these are distributed as patch files that don’t contain any Nintendo assets and you have to provide your own ROM and patch it yourself.

Where people get in trouble is when they try to make a full game (as opposed to a mod), like that Metroid game that was shut down awhile back.

Answering my own question: Yes, both are included in the mod.