Sprites vs. Portraits - Creative License
As an artist, I would like to defend the creative license required when creating sprites for characters, enemies, and bosses. But, you are the player--and your opinions are more important than mine!
So, let me make my case. It is very clear that the mediums between the two are different. Portraits (of the face, or entire body of a character) are much larger than a sprite--in this case, the sprite is anywhere from 1/4 to 1/8 the size of the portrait. Sprites a very small, and every single pixel must be used to convey the form, motion, and texture of the creature in question. A portrait is much larger, and has space for decoration, trinkets, accessories, interesting clothing and the like.
It is only natural that the sprite will be a diminished version of the portrait--both in size, and in detail. Not only are the details smaller, but many of them will simply disappear. If there is a decorative lions head on the character's shoulder pad, it is likely impossible to properly show the decoration on the sprite. If you'd like to dispute the point, try it for yourself!
The lingering question is then--should the artist create boring (you could also say simple) portraits of characters and bosses so that the accompanying sprites can match the portraits, or is it more engaging to see flashy, exciting portraits full of detail and intricacies even if much of that detail is lost as the character is boiled down into a sprite?
[[ Attached is an example of one such simple portrait and sprite. All of the details in the monster's portrait can be copied into the sprite without dropping anything. But, the desert raider wears simple garb, carries a simple weapon, and purposefully has no details that cannot be transferred. ]]