Triminac's mutation into Malacath coincides with the Orc's mutation into orcs, but Malacath could probably make himself look like he has whatever nose he wanted to have.
That's of course true, while the origins of the Orcs and Malacath may be interconnected, I don't think there's anything that forces Malacath to look the same as them.
But in any case, I believe it's as Alaisiagae said, the facial features of the Orsimer we see now are carried over from their origins as steriotypical D&D monsters, I suppose it's a similar situation to elf ears, I don't recall reading anything about why they're pointed in the lore, they just are because that's how elves usually look in fantasy, even if there is lore explaining where the ears or the noses come from, it would just be something thrown in later on to explain something that is really purely a design choice.
As far as I know, we know that what we see is how they're supposed to look in the lore, but I don't think we're told why.