Setting inconsistencies aside, I've reached the conclusion that Grimlocks nosed face is an aberration in the tfp universe. (Seriously, it's not a joke, im not saying this only because he looks terrible).
Shockwave saw the dinosaurs and thought to reproduce those in cybertronians. We do not actually know how long it took him. Perhaps he also happened to watch early humanoids and thought that he will find out if he could add that to his experiments too, because why not.
To Cybertronians, the nosed face of the dinobots (but especially Grimlock, because looks so terribly human unlike all the others whose faces are metalic smooth) would be as unsettling as for humans seeing someone who got their nose ripped off. It is another visual evidence of the horrifying things that Shockwave does, as if being turned into a weird alt mode and having a messed up processor was not enough. Hence why Grimlock is not fond to removing his face plate, why his face is actually torn and having holes which let his internal components exposed under it, and why he seems to have trouble to be lucid without it.
