As a creation that was man made, not heavenly created, Frankenstein the monster has no soul. Shelley is very clear to illustrate this. He is not "nice" or "mean" but merely full of purpose. He struggles not with a moral code that governs most of human society, and feels no remorse for the crimes he commits. He is apathetic to the world outside of him. Shelley was viciously against science for science's sake, and wanted to portray this tragic character as lost, attempting to fit into some society, but really not a part of any.
Again, if you are referring to the scientist, some people might call him mean because of the way he rejected the creature after he created him. His first reaction to him is in terms of his looks: "his yellow skin scarcely covered the work of muscles and arteries beheath; this hair was of a lustrous black, and flowing;....his watery eyes...his shriveled complexion and straight black lips." Frankenstein runs from his creation partly in horror that the violated rules of nature but also because he was so ugly, so horrid-looking to begin with.