Boris Karloff did not receive an Oscar nomination for portraying the monster in Frankenstein.
Boris Karloff was actually credited as ? although I have to think that perhaps that he why he eventually started to be billed as KARLOFF to try to make up for that. Karloff plays here his most famous role, the role that made him famous, and the role that just was not sexy enough for Bela Lugosi. Where the Bride of Frankenstein focused mostly on the growth of the monster this film, which depicts the "birth" of the monster, focuses much more on Dr. Frankenstein (Colin Clive). In fact Karloff could very easily be considered supporting in the film due to his screen time, although the monster does not support anyone else when he is on screen.
Karloff becomes the star of the picture the moment he lumbers out of the darkness. Karloff just like Lugosi did in Dracula creates an image that is one of the most iconic images in cinematic history. Of course Karloff wears considerably more make up then Lugosi to make the monster but having the make up does not make one the Frankenstein monster. Karloff's physical performance is ingenious in his creation of the manner of the monster. He moves as if he is a man made up of various body parts. He has a disjointed walk as if he is not comfortable in his own skin and Karloff perfectly represents exactly where it was that the monster came from in his performance.
In the first film Frankenstein does not speak rather instead he only makes noises. Rather then giving the monster some sort of intimidating growl Karloff again takes a far more interesting approach by portraying the Frankenstein as more of a child incapable of words. Karloff vocal performance along with his physical one does not make the Frankenstein some evil creature but rather honestly what was crated. Karloff makes the monster something that is both old and new and altogether wrong. He is old carcass in his body but in his mind Karloff properly shows is something that has just been born.
One of the memorable parts of his monster, which is most forgotten in later portrayals by other actors in the role of the monster, is the strange humanity Karloff brings in his depiction. Karloff's best scene in the film comes when the monster is invited to throws flowers onto a lake by a little girl. Karloff does falter once in a scene that easily could have seemed silly but instead is quite a poignant scene before it turns dark. Karloff brings such an honest wonderment and joy to the monster in the scene. Karloff believably makes in the monster a tender soul that lies within its twisted exterior.
Boris Karloff as the monster would achieve greater heights when he went on to reprise the role in Bride to Frankenstein where he was given an actual character arc for the monster as well as even lines as the monster learned how to speak. This is a simpler version of the monster but what Karloff does with the role in the creation of the monster is remarkable. Karloff's performance as Frankenstein monster is the definitive one for the monster. His performance is not just being a ghoul and trying to scare someone merely through his appearance but rather give a sympathetic portrayal of the travesty of nature created when a man tries to play God.