fashionable, popular, trendy, and common all mean different things even if they are somewhat interlinked concepts. High fashion then stands even more apart from the rest.
I kinda like this Cambridges definition[1] of fashionable more (even if isn't perfect)
> wearing clothes, doing things, and going to places that are considered stylish
There is clear difference of what is considered stylish and what is popular.
Macmillian[2] has even more poignant definition available:
> popular with rich and successful people, and often expensive
Which I might disagree on specifics, but it still carries the thought over. Fashionable is not (necessarily, or even commonly) popular with the common people, but of some sort of elite.
The important aspect of what.you describe is that fashion is more aspirational than popular.
A lot of what is "popular" is still recognized by the wearer themselves as not fashionable, and is worn despite that (and sometimes because of that) for various reasons.
That's kind of what fashionable means.
Oxford dict:
Fashionable - characteristic of, influenced by, or representing a current popular style