This is an interesting question! The simple answer is yes, it was extremely deliberate.

The more complicated answer is that it was also organic. Punk as it was in the beginning quite honestly had a limited lifespan. It began as a reactionary genre against a very specific set of circumstances, namely the near impossibility of being signed and supported if you didn't fit a certain look and sound. That's where the DIY aspect of music production and distribution originated and the more general DIY aesthetic followed naturally. It was a bit of a "form follows function" situation wherein early punks sought to differentiate themselves from the mainstream and they did so by taking fashion to the extreme. In many ways it became just another part of the brand.

In the late 70s and early 80s (largely in response to punk), labels started to broaden their repertoire and the conditions that the genre started in weren't really in play so much anymore. That meant it lost the context it made the most sense in. Without that context and with the ever-increasing influx of people drawn in large part to the DIY part of the scene, it was a natural evolution for punk to split into subgenres. This is where post-punk came into play.

As you know, it's musically much more complex than punk. What you might not know is that it incorporated a lot more modernist art and intellectual aspects as part of its overall aesthetic and production. This meant that in a lot of cases, going to a post-punk concert could be more of a contained artistic experience rather than simply seeing a band play. The renewed fashion aspect of post-punk can be attributed to this difference. In part it was simply another piece of the performance, but in part it was also a natural evolution that came from the kinds of people who were drawn to making the music.

While punk had been very raw and rock'n'roll oriented and drew people who fit that mold, so to speak, post-punk with its more intellectual bent tended to draw a more.. refined.. sort. The often more academically leaning folks who populated the post-punk landscape brought that flavor into the aesthetic and fashion. That said, there was a lot of derision toward the punk aesthetic that came before. Because of that distaste for the rawness and wildness and the desire to make it known that while there were similarities between the old and the new, there was also a deliberate decision made to dress in ways that would clearly set post-punk bands apart from their predecessors.

Feel free to let me know if you'd like any clarification on anything. :)

Sources:

Rip It Up and Start Again: Postpunk 1978–1984 by Simon ReynoldsBabylon's Burning: From Punk to GrungeAgainst and Beyond: Subversion and Transgression in Mass Media, Popular Culture and Performance edited by Agnieszka Rasmus

(Edited to fix formatting, the bane of my existence.)