I don't think some storylines require or are worth the time and effort of creating radically different and unique aliens.
A lot of the time a generic 'forehead of the week' design will suffice.
Inevitably when faced with seven years' worth of creating new designs for alien races some will be more radical than others - for every Species 8472, Hirogen or Vidiian you'll get half a dozen Taresians or Krenim-type aliens.
I wouldn't call it laziness as much as the reality of weekly television production.
However, most of the aliens we did see in Voyager were full face laytex.
There wasn't one forehead alien in "Fair Trade"
Like him or not, the work and design that goes into creating Neelix alone is one of the most complex made for a regular cast member IMO. Painting on the spots alone is time consuming, not to mention the laytex and hair. He done so well, you can't tell where the laytex ends and real skin starts. You could always see that flaw in Worf's make up