Greg Bear wrote a Trek novel, Corona, early in his career, just before his fame really started to take off. David Gerrold got his start writing Star Trek on TV and then went on to become a widely acclaimed author of original SF; he wrote one original Trek novel, The Galactic Whirlpool, for Bantam in 1980, as well as the novelization of TNG's pilot "Encounter at Farpoint." Other, more prolific Trek authors who are also well-known and highly regarded for their original work include Diane Duane and Peter David.
However, perhaps the most famous, important, critically acclaimed science fiction author ever to write a Star Trek novel was Robert Sheckley, but his Deep Space Nine novel The Laertian Gamble is widely despised as not feeling a lot like DS9. Similarly, K. W. Jeter is a well-regarded SF novelist, but his DS9 novel Warped just didn't feel right and was so unpopular that it killed interest in hardcover DS9 books for over a decade.
So people who are acclaimed for their original work won't necessarily produce acclaimed tie-in literature. It's a different discipline with different demands. It's necessary to balance an original voice and perspective with fidelity to the voice, characters, and continuity of the work you're tying into, and authors who are used to doing their own original work can't always make that transition -- can't always adjust their own voices and sensibilities enough to produce an authentic and satisfying tie-in. (No value judgment there; it's just that not every artist can adapt to multiple disciplines. Screenwriters aren't automatically novelists, painters can't necessarily sculpt, and being a great violinist won't make you a great sax player.) So it's unwise to assume that fame or reputation alone is the only measure of quality here. There have been some brilliant works of Star Trek literature published over the past dozen years, but few are by anyone who'd be considered a big name in broader literary circles (at least, not yet, he said hopefully).
