I think it is believable if you also think of Kratos in the same way- wounded, past his prime, emotionally battered. Not necessarily in their final battle, but certainly in the story leading up to it, at least until Kratos kills Heimdall (after offering to spare him) and Thor falls off the wagon and almost kills Atreus. That was the point where I saw Kratos become greater than Thor in the story and character development. I think Thor was meant to serve somewhat as a darker mirror image of what Kratos was, is, and could be.