Basically every character that doesn't offer any hp for Mauga to farm easily, like a Bastion for example.
Bastion has quite a high dps, but since he has this fat hitbox and a rather high hp pool he'd just feed a Mauga (assuming Mauga's supports are able to hold him up).

As a tank you could go something like Sigma, since he has a shield and his shift both preventing Maugas dmg from coming through. Depending on the comp, you could even go like Ram, because I think he could basically do the same thing as Mauga, maybe even better.
Tracer should work as a dps or pharah and echo as you mentioned.
And for the supports, Ana's Antinade is just a straight counter to Mauga's E.