Just as Mamenchisaurus is said to be a Diplodocus-type sauropod, it has a long whiplash tail as a defensive weapon against predators. Furthermore, Mamenchisaurus is one of the world's longest dinosaurs. In Jurassic World: Evolution 1 & 2, the body and posture looked nearly similar to Brachiosaurus or Dreadnoughtus.
Apatosaurus and Diplodocus are the best representatives of the Diplodocus-type sauropods. Therefore, Mamenchisaurus should have a similar posture to those two. The way Mamenchisaurus was presented in the movie adaptation of The Lost World: Jurassic Park matches those two species.