Mainstream means popular, sort of. It means normal, expected, commonplace.
If you’re outside the mainstream, you can see it more clearly. You can even swim against it if you don’t like where it’s going.
It’s a great metaphor, but it’s also a trick. Just using the word mainstream implies that you aren’t in the stream, that YOUR art or music or political opinions are special and different. Mainstream things are for people who drift helplessly with the current. Not you.
Maybe you are more thoughtful and refined than the average person. Someone has to be. But don’t flatter yourself with this story about the stream, and don’t let anyone else flatter you with it, either. People who make a big show of saying their ideas run counter to the mainstream are people who lack confidence in their ideas.