These things happen. TMP featured extensive and elaborate alien makeup and costumes. Didn't see much of it at all. Far background crowd at SF HQ and then again a crowd scene on the Rec Deck. A few standout characters but nothing representative of the array of aliens they worked on. A lot of effort for comparatively little screen time.
Weren't a few of them already developed for the aborted TV show?
Also, as far as costumes, I think it was a bit of Fletcher getting the keys to the old paramount art house store room and given a "blank check" and carte blanche. He seemed to gcream overboard, creating whole back stories for the aliens he worked on, using thousands of dollars worth of antique fabrics and jewelry that were just sitting in storage not having been touched in twenty to fifty years.