It is more than just nudity. This is the formal rule, note the emphasized part.
> NSFW content is not allowed. Please keep it safe for work and avoid anything overly edgy or distasteful.
There was some additional discussion about this in GBCompo 2021in the following issue. More context in the link, but here's the relevant excerpt:
https://github.com/gbdev/gbcompo21/issues/9
> If it meets ESRB-Teen then it should be fine. https://en.wikipedia.org/wiki/Entertainment_Software_Rating_Board#Ratings
> In general, if it's relatively tame and isn't trying to "push the boundary" it's probably ok. As the saying goes- it's hard to define it precisely (and we don't want to enumerate every single thing), but you likely know when it's getting iffy.