Today, many works of prose tends to use a style of expression that tends to mimic the general language used by the modern reader. This kind of style is most prevalent among what can be referred to as works of "general" or "popular" fiction, which are read for their ability to entertain. The Twilight or Stephanie Plum novels come to mind as examples.
For novels such as these, a phrase like "the silver pepper of the stars" would certainly appear out of place at best, or even pretentious at worst.
On the other hand, there are also genres that seek to serve a slightly more intellectual audience. These works are based not only on entertaining plot lines, but also on a somewhat deeper interaction between character, plot, and the resulting growth, destruction, or combination of these. Many of the novels we consider "classic" today take this shape. For the modern reader, works written during the early 20th century appear to be particularly accessible within this genre.
The linguistic style used in these words tends towards the poetic, with authors apparently effortlessly weaving imagery and sound into a work of art that is far deeper than only entertainment.
The Great Gatsby by F. Scott Fitzgerald is no exception. The "silver pepper..." phrase is by no means the sole example of its kind in the novel. Had it been so, it might have been referred to as "pretentious" or "inappropriate" or any of the other derogatory terms a reader might choose for the purpose.
In Gatsby, however, the phrase contributes to a larger work filled with similar expressions while also managing to tell an engaging, entertaining, and moving story about a man who believed that his dream was Daisy.
In fact, phrases like "anchored balloon" to describe a couch and "a loud bright night with wings beating in the trees" to describe one of Gatsby's parties keep good company with the pepper of the stars. The prevalence of these phrases contributes to the overall tone and style of the work.
Furthermore, the poetic language that Fitzgerald uses to describe Nick's observations serves a deeper purpose. It is symbolic of the apparently desirable glamor associated with Gatsby and the crowds that always surround them. The words sound beautiful, but they describe stars, which, while beautiful, are not an uncommon sight. Daisy and Gatsby and the rest of their friends are beautiful, but ultimately the novel reveals them all to be very human, just like Nick and just like the reader.
I would therefore argue that the "silver pepper of the stars" is both poetic and necessary, without which the novel would not be the rich experience it is today.