This is probably the only passive game you should agro but a good way to tame one is to agro it on a flyer preferably a pteranodon they do not much damage and are fast have mejos in ur hot bar and fly above it it’ll just stand there trying to hit you with its head and then dive In on it when you see the orange meet coming of it’s face and feed again easy peasy
When taming to make it easier stand against a cliff or a good sized rock. I tamed one for my brother and it was a piece of cake with that technique. Also if you find that the place you find the diplodocus isn’t safe just get near it and lead it away. They won’t hurt you they just like to swat you around like they are playing a game of soccer.