I think that if they somewhat downplayed some of the f/x, it might have gotten an ‘X’ rating, for an odd reason.
The movie Angel Heart was initially given an ‘X’, because of a particular scene in which Mickey Rourke was having sex with Lisa Bonet in a derelict building, during a rainstorm where water was dripping through the ceiling into pans.
The scene cut back and forth between the couple and the pans, and was deemed “too intense”. But by cutting less often to the pans, it could get an ‘R’ rating. They actually got an ‘X’ rating not because of the sex, but because of old dinner pans with water dripping into them.
Much the same rule might apply to The Thing, because the scariest parts of the movie don’t actually show the monster as such, they just imply the monster is there. The f/x are so over the top that they’re almost comic relief.
Had they toned them down, it might have been “too intense”, as far as the MPAA was concerned.
I remember that the biggest criticism of the movie back then was specifically for the f/x. I could never understand the 1-star rating the movie got on my cable company’s TV guide even as they were showing it 20 times a month.