I think your number is too big, home many nude mods can be for a game? If you mean some revealing armor/clothing then that is not porn not even "adult".
There will always be a large number of lower effort mods, it is easy to retexture or cut parts of existing armors then building new ones from scratch.
The main issue I see with some modding communities is the big egos of some people that leads to terms of use like "don't share this, this mod should not be shared on as different website" , it will make it kind of illegal then when the specified website will no longer exists to archive and move the mods or to create mod packs.
But obviously it depends on what games you're interested in seeing mods and fan projects of. The vast majority of the Mario/Zelda/Sonic/Metroid/Mega Man fan game and game mod/hack scene is basically entirely new games built on the original series' engine.
Some of the stuff you see there is crazy, arguably on par with many of the more well funded open source software found online. Like that project which adds online multiplayer to Super Mario 64, the mods which add MP3 music to SNES games, a Metroid mod which remade Metroid Fusion in Super Metroid with all the original content and an even smarter version of the SA-X hunting you down. Or perhaps Wiimmfi, which basically replaced all of Nintendo's online services for the DS and Wii, then significantly expanded on them with extra features and modes.
It's kind of insane how technically advanced much of this stuff can be, given it's all made by hobbyists in their free time with no hope of ever getting paid.
I'm mostly familiar with EV Nova and KOTOR mods, which are posted to websites that probably filter for obscenity. EV Nova doesn't really have pictures of people; but KOTOR definitely has plenty of appearance-focused mods (including one described as "More sexy underwear for females"), and on some other games I have heard of mods that fall into the "weird porn" category.
