After the first season fewer and fewer aliens from the Minor Races were seen. I suspect budgetary restraints played as big a part as a tightening focus on the story arc: not introducing new aliens I understand, but whatever happened to all those other ambassadors? Nevertheless J Michael Straczynski plausibly explained this as intentional:
We’d like to get away from the alien-of-the-week story … and concentrate on the aliens we already have. If you just keep on throwing new aliens into the mix, soon it loses all impact. We decided to try a whole bunch of aliens over season one, and pick the ones that worked, which we would then work to refine and integrate more fully into the storyline.
The makeups were originally designed by John Vulich and Optic Nerve Studios who explicitly set out to lift the bar for Sci-Fi TV shows: ‘not just a forehead’. They largely succeeded – setting a standard which only Farscape has beaten. The Narn makeup was particularly ground-breaking proving it was possible to put leading characters in neck-up prosthetics within the time and budget constraints of a TV series – although there were problems in finding actresses prepared to wear it.