Tarma's asexuality is indeed canon; it's not just part of her oath, but an actual gift from the Star-Eyed, because Her representatives cannot afford the distractions sexuality would cause. In one of the later stories, it's also made clear that "asexual" is not the same thing as "without love" -- Tarma has to recognize and acknowledge her love for Kethry in order to save Kethry's life.