She is actually a good character, but suffers the same fate as most of the defense category: bad PR. There is a long standing perception that the defense category was inferior to the offense category, as offense characters could be used effectively on defense as their mobility compensated for their lack of area denial abilities.
The dev team has been addressing this one character at a time, first by taking an OK Junkrat and increasing his mobility and kill consistency. This effectively overpowered him, leading to an increased pick rate and dispelling the F tier stigma. They are in the process of scaling him back. We’re seeing a similar process with Hanzo currently.
I think they thought the freeze buff would have a similar effect for Mei, but it hasn’t materialized. I think the next character up for an unnecessary buff is Torb, another character who is much better than perceived. Mei will probably get something more then.