According to Paula M. Block and Terry J. Erdman’s book “Star Trek: The Next Generation 365,” makeup designer Michael Westmore wanted to make the Romulans look more threatening.
Westmore said, “From the very first moment [Romulans] appeared on-screen, the viewer had to take them seriously, rather than seeing them as stereotyped villains with pointed ears.”
The designer used the forehead to express they were villains and different from Vulcans by “giving them a built-in sullen expression they couldn’t get away from.”