First off mugen is not a fighting game to begin with it's a customizable engine for fighting games.
Secondly, you may put what ever content you want in this engine if users aren't comfortable with 18+ content then don't download it...don't remove it from the site entirely, it's not necessary since the site gives warnings about the content being mature/adult content.
Thirdly, Everyone is allowed to put whatever they want into their rosters. I have an entire 18+ roster and then a safe for work roster for posting videos with. Your roster your rules, your choice and your FREEDOM! Holy or not the user in question may have been referring to his mugen being holy? if not and may have meant to be for all of mugen to be "holy" i mean it's fine..it could be by his rosters standards but do not oppress others for their choices...religiously, culturally or even socially...these choices are offered to us everyday we choose and we make our own rules but these said rules do not apply to every server or site you go to, respect that and move on. do what you do. For me i make large variety rosters and i love joke edits but i tamper with 18+ stuff at times and i enjoy naked females fighting each other or some hentai monsters.
all in all Mugen united us as a community for our love of what the engine does and what it offers not by the rules we make for it but for the content we download and appreciate everyday.