IMO, Scary onscreen and scary in real life are two different things. I thought the scene in Paranormal Activity where the hapless couple saw footprints in the flour on their bedroom floor, but then realized what they didn't see, was scary... in a a fun, "creepy once you realize the implications," sort of way.
But if I'd actually been in that room, and convinced (as they were, by then) that there was something demonic in their house, I'd have felt something much more intense, and not at all "fun."
So did I ever think GMK's Godzilla was 'scary', in a cinematic way? Yes... but what made him scary only partially derived from his appearance: specifically, from the brutal jaws and his ghost-like eyes.
In combination with his head, what really made him frightening (again, movie-style frightening) was his behavior: like letting Mothra try to sneak up behind him, letting her think he was preparing to blast Tachibana's ship, out in the bay, then turning at the last moment and blasting HER: fatally, as it turned out.
Or staring down at individual human beings, and preparing to murder them with his atomic breath.
How many times in other movies has Godzilla even noticed unarmed human beings, much less paused to make certain he killed them?
One scene in G vs KG comes to mind, but even then, he didn't come across as calculatingly evil, like GMK's Godzilla does.
"Rudeness is the weak person's imitation of strength." -- Eric Hoffer