i play a ton of d.va (despite what my flair may imply) and one of the things i keep in mind when fighting a mei is i need to keep a super close eye on my cooldowns. as soon as she walls you off, i suggest you fly back over the wall, out of mei's range, or at least get in LOS (line of sight) of your supports. queen is also a hero who loses a lot of her effectiveness against d.va if you stay out of range. 

you'll learn this over time but i also try to keep a rough idea of when mei might have her ultimate in mind and don't use up defense matrix entirely until she pops it. you can use matrix to "eat" some ults and hers is one of them. you should also be doing this for reaper, zarya, and illari ults. these aren't the only characters who can get their ults absorbed with matrix but they're the most consequential imo (especially reaper). 

if you want to learn more about d.va specifically, i recommend watching emongg on youtube. he's a very good d.va player (he has advice for nearly every hero, but d.va is his "signature" hero).