People refer to a lot of things as being part of pop culture. Sadly, most of the people that use this term do not understand what it means. Pop culture or popular culture is fairly inclusive and can refer to a lot of things that have the same attribute: the dominant belief, practice, or attitude towards an object, trend, line of thoughts, etc. Pop culture goes beyond local culture and is considered to be a wider phenomenon.
Like most other trends, fashion is part of pop culture, and elements of pop culture can influence fashion. A good example would be clothing collections that are inspired by popular TV shows or personalities. However, the trend works both ways.
We see personalities adopt a certain trend which then is adopted at a mass scale, becoming a part of pop culture. A good example would be the parachute pants popularized by MC Hammer. Prior to these types of pants being used by the musician, the fashion trend was relatively unknown. It became popular after its association with MC Hammer.
The above two examples show how the fashion industry and influencers or personalities create trends that become part of pop culture. In most cases, a fashion trend becomes popular once a personality brings it to the surface. The fashion industry itself is well aware of this effect and sometimes collaborates with influencers to push a new fashion trend.
Not all of them manage to be adopted at a mass scale and become part of pop culture. For this to happen, fashion products, clothing, and accessories need to be affordable. You would rarely see designer clothes become part of pop culture. What happens most of the time is for fast fashion to adopt design trends inspired by designers. When this transition occurs, fast fashion becomes closer to what we all consider part of pop culture.