Pisces is a very soft-edged sign, and prefers their style to be as laid back as possible.
Clothing that is soft and flowy connects you to your Water element, and reminds you to go with the flow in your personal life as well. Overly constrictive clothing just makes you feel hemmed in and uncomfortable.
Even if everyone else is wearing it, you need to make it your own. You won't look good in even the most flattering outfit if you aren't feeling good in it!
One way Pisces can wear trendy but not-so-comfortable clothing may be to tailor it to your own body. You can make the styles of the day fit you, instead of the other way around.