I agree. At the very beginning of the series they probably never thought that they'd show Spock's parents, so why not assume that full-blooded Vulcans looked more alien?. There are always interesting things like that in a series before they've completely figured things out.
This, and "Balance of Terror" scored a plot point (the "bigotry" bit) by making the Romulans look just like Spock. And BoT explained it by saying the Roms were a Vulcan offshoot. Once they did that, the "real" Vulcans coming later ("Amok Time") would have to look just like Spock. And that conflicts mildly with Harry Mudd's keen eye for ethnicity. Not a big problem.
What I object to is the TV spinoff-era Vulcans and Romulans having ugly, built-up foreheads like cartoon cavemen. It's a continuity clash with TOS, and the later guest shots by Spock and Sarek on TNG would of course not have the forehead appliances. More clash.