I wouldn't say she's a better tank, no. She can potentially survive an infinite amount of damage, sure, but only when ice block is up, and she can't do anything during that time. She doesn't force enemy movement the way Roadhog can because Mei is dangerous within the range of her primary fire but Roadhog is dangerous within hook range which is far longer. A team can't rally and push behind an initiating Mei the way they can with Roadhog because his long term survivability is better and the danger hook poses is greater. Mei is mostly good at splitting teams in half with her wall but that's not reliable in every situation