Sigma has been the correct choice against Mauga since his release.
I only use Zarya and he’s not THAT bad. IT more of a skill match-up in this case. Neither really counter the other. If Zarya is too agressive he just eats her up like Bastion does. If he is too aggressive then Zarya gets some good energy, especially since she should be playing longer range and corners, so she can try to get him to break LoS from his team.
Otherwise its kind of boring, just poking eachother.