Well, it is a bug.
Even man in charge of balance has acknowledged it:
Mei could snipe Widowmaker’s, but if Mei was losing the battle, then she could spawn a wall to hide behind it. McCree had to run/roll away.
Mei could heal herself and be invulnerable. McCree not.
I’m not certain but I believe they also increased the cast time with the same patch that gave her falloff damage in the beta.
But I agree with you on this one:
She’s a hero all about crowd control. Because of this is she best used in close to midrange battles, but even then are there heroes who can deal more damage and are mobile than Mei in the same range. Junkrat being one of them.
What’s also interested is that they have tried giving Mei mobility but eventually decided to make her a counter to it: