You know sniping her helps but also heroes that can CC her, or overdamage her with superior firepower. Orisa, Junkrat can both fare well against Mei.
Or you know. A good Pharah. Walls are of little use against a flying angel of war. You can knock her off of your squishies, you can slap her left and right with splashdamage, force her to ice cube and just chase her down after. If you’re flying dynamically and out of her reach, Mei can do literally nothing against Pharah.