It isn't Skyrim level of modding, but there are good mods for it. Like random encounters and school of the roach is epic. plus some nude mods.
However, I'm personally not playing Witcher 3 until someone replaces geralt with a female :P I might be a guy, but I don't like playing males in games generally (like back in WoW time, a lot of the horde races made epic males).
But witcher 3 is just geralt and only geralt. I know thats what the story is and what not, but I don't want to play geralt :( I'd rather choose to be a female lol.
If it wasn't for that though, there are some pretty decent mods. However, modding witcher 3 is vastly harder than modding Skyrim or fallout. The file system for one is WAY harder than what Bethesda uses, which is much simpler and anyone can do it. Bethesda makes every file easy to find and named something easy...witcher 3 the file names don't even make sense and you need a ton of programs just to unpack them and stuff. And even when you unpack them, need special programs to open the unpacked files. It isn't really a system designed with ease of modding like skyrim or fallout 4.