Normal Amaterasu would be horrible to use against Mokuton. Amaterasu takes a considerable amount of chakra and the largest Amaterasu we have seen was from Sasuke against the Hachibi which is incredibly small in comparison to Hashirama's top Mokuton techniques, especially SM Hashirama's Mokuton techniques. So essentially you'd just be wasting chakra to use Amaterasu.
Yeah you can argue the Fire will spread given enough time, but I fail to see how that helps someone facing Hashi, it would just leave the enemy in a sea of Mokuton and Amaterasu Flames LOL. They'd kill themselves lol. While Hashirama can generate Mokuton faster than the fire can spread.
Now you could say what about using Amaterasu on Hashirama directly, but Hashirama can just push the flames off himself by sprouting Mokuton from his body, while at the same time healing himself. Heck he might be able to blast the Amaterasu flames off himself by just flexing his chakra, the dude shattered concrete with the force of releasing some of his chakra and that most likely wasn't his max even in base let alone SM. Also this is assuming someone could even get the chance to utilize Amaterasu why having to dela with Hashi's borderline OP Mokuton Jutsu, Moku Bushin, Bringer of Darkness, & w/o Hashi sensing Amaterasu pressure and putting up a shield.
Also just general portrayal of Hashi should tell you the Amaterasu we have seen so far is not going to be much of a threat to Hashi. I mean Kishi had the dude fighting Kyuubi-Susano'o fusion, so to expect Kishi to show any Amaterasu usage we've seen so far to be effect against Hashi, unless Hashi is in some extremely weakened condition is simply unrealistic.
Granted if someone was able to generate Amaterasu on much much larger scale than it could be effective, but so far no one in the manga has proved capable of that.