I'll say this much for Mego's figure sculpting around 1979: at least McCoy looks like DeForest Kelley and Scotty has a very good resemblance to James Doohan in the first movie. Most of them looked pretty good considering the limitations of action figure design and technology at the end of the seventies.
I won't say Decker looked just like Stephen Collins, but it was a fair resemblance all things considered.