They changed Murtagh's appearance quite a bit from the source material to the show — fitting, considering they cast Duncan Lacroix in the role. Murtagh is supposed to be a short, ugly man in gross, ill-fitting clothes. He is often described as rat-like with simian appendages; essentially, Murtagh is supposed to be mismatched and unattractive.
Duncan Lacroix is not those things. He's 6'2", and has a well-kept beard and head of hair on the show. His appearance isn't the only thing that changed from the Outlander books to the television series, either. The showrunners changed quite a bit of his story as well, giving him a much-expanded role and allowing him to survive past the point where the character dies in the books.
Not too many actors are rat-faced, and Lacroix definitely is not. He often appears on the show in clothing that is not as nice as some of the main characters, but that's about all you can say as having the television character match the physical appearance of his novel counterpart.
