The game is rated for Adults Only due to explicit sexual content and nudity.
As for modding, while I Got a Millenary Cat does not officially support mods, the fact that version 1.2 is distributed as unpacked assets suggests that are theoretically possible. A few fans on Baidu Tieba have already shared modified sprite packs, though there is no centralized modding community. I got a millenary cat Free Download -v1.2-