I think that part of the problem is simply the different shape of the head. Unlike humans, the ponies have muzzles/snouts. They aren't as prominent as they would be on an anatomically accurate horse, but they're still there. And once you add that, all mouth expressions change enough to be non-transferable. The animators use the standard cartoon trick of keeping the mouths as human as possible when feasible; giving them human prehensile lips, and usually not showing most of the mouth (if you think about it, their mouths actually extend all the way back to more or less parallel with their ears so there should be a lip line running along pretty much the entire length of the bottom of their face.) But the mouth/eye line is STILL different than it would be on a human head; it HAS to be. So the expressions are NEVER going to match up all that well.
And no offense, but the mallet one is just dumb. Human rarity has HANDS, why would she hold the mallet in her teeth? (actually, since Rarity has magic, why would she hold it in her teeth anyway).
Rarity's mane design doesn't really translate well to human unchanged either. You note that, in the full length pic, trinityniyang basically had to "split" Rarity's mane to make it look sort of normal (that is, the curl that is normally on the bottom of her mane is now on a different set of locks than the one on the top. Rarity's mane style is very, very thin (since the actual scalp is pretty thin). On her in Pony form it looks okay, because we are used to horses having manes that are kept very thin and tight. But on a human, there'd be too much bald scalp. Rarity would look like she had the worlds longest Mohawk.
"For I have heard the Song of War, and to me, it sounds just like "Please, Mister Custer.""
Cogito Ergot Sum, "I think there is something wrong with this rye bread." (Aplogies to Seafrol.)