Though it’s not wrong to follow fashion trends, I suspect there has to be a reason why others are told to avoid it or at least minimise on depending on it for so much. One could easily blow thousands of money on making your own clothes instead and that of others. And the odd fact that for most of the part, folk and clerical clothing don’t as much or at least as drastically as secular fashion trends do (though in some cases, when it comes to missionaries, the fashion sense gets changed for the better on their behalf*).
Nun clothing has remained fairly similar** over the ages even though it’s changed in some cases. After if the principle’s to avoid what’s hated or forbidden in your religion, it’s better to stick to what seems dowdy but effectively distinctive. (I think even some religious texts have a similar idea so.) It’s not wrong to follow fashion trends and it’s not wrong to be influenced by anything else. The real issue’s that either you need to be pickier with your influences or reject trends for good.
*Ireland’s also an odd exception but in that it’s got to do with economic reasons though some Irish dance groups still stick to traditional costumes more.
**Similar things can be said of football/soccer uniforms and to some extent, certain nursing and doctor uniforms too.