Is lockjaw always fatal?

No. First of all, tetanus is relatively rare and has decreased in incidence since vaccination began in the 1940's. In the us, mortality rates recently have been around 15-20%, compared to 91% in the 1940's. It is much lower if one is vaccinated, with mortality rates around 6% if vaccinated. So get that vaccine or booster if you haven't!
NO. Most patients with lockjaw die from complications not directly from the infection.