To be non-mainstream means to show individuality, to be alternative, to be non-popular, not to blindly follow the current popular trends, but to pay attention to clothes, clothing, words and deeds that are in line with one's own character.
To be non-mainstream means not to be the most popular. Popular things are relatively advanced or lagging.
In terms of numbers, people who pursue non-mainstream things account for a relatively small number. If there are more people, they are called mainstream.
Where there is mainstream, there is non-mainstream. Mainstream and non-mainstream cannot exist.