The lemon term originated in manga/anime fandoms and I've been told by people in that side of things that it actually originated among native Japanese-speaking fandom before migrating to non-native speaking manga/anime fandom and eventually the rest of fandom.

Google seems to back that. Looks like there was a soft-core thing in the 70s called "Cream Lemon" that resulted in "lemon" becoming actualfax Japanese-language slang, which was adopted by people in fandom as a handy descriptor/rating.