(we do know pretty much precisely what he looks like, though, if all male grineer are made from variations on the same template. It's not--look, let's call them homely)
A lot of the grunts don't have faceplates, so we know what they look like. They're hit with a triple-whammy of fast aging, susceptibility to skin disorders, and no one taking care of those facts.
Tyl might say more than most about what the grineer are lacking, but his only big complaints are 1. the average one lives too short 2. they're not as smart as me
grineer women are split between height extremes but are uniformly more overtly mechanical, with straight elements or details that make their cybernetics obvious. Males are kitted out with more curves. Heavily built up-top for most of them, but they've got more prominent hips.