Mei a problem for Zarya? Just wait for her ice build up, shield, and get away. You can also shield out of her Ult. I've never had a problem with Mei as Zarya; I can usually get away from her and then the rest of the team just focuses her, or if I'm mid/high energy I just melt her.
Competitive isn't about having fun, so how fun D.Va or Junkrat are is irrelevant; it's about being effective. Most people taking Junkrat for a spin in Competitive do so in situations where he isn't good/are not effective as Junkrat. Same for a lot of D.Va players. With that said, I think D.Va is a good hero for competitive, and have had some D.Vas that were a headache on the enemy team, but I can hardly ever remember a problematic Junkrat on the enemy team that wasn't easily shut down.
The newbie's "Junkrat dream" is that they're gonna lob grenades, get MAD kills and ult charge, and detonate that GAME CHANGING HIGH OCTANE RIP-TIRE.
The reality of most competitive Junkrat players is the opposing team just avoids/swallows your grenade with D.Va's Defense Matrix /Zarya's Projectile Barrier/Rein's shield, avoid your traps and concussion mine's because they're easily spotted, and you'll be killed half way into your ult because you have to stand there, vulnerable. And if your ult gets any good kills, it'll probably just be insta-countered by the enemy team's Mercy res.
I swear every time someone picks a hero that is a terrible choice for the situation/map, they think they are just gonna KILL IT because they have this MIND BLOWING strategy that YOU JUST CAN'T WAIT TO SEE, and it falls flat. Pick what works, get good at meta heroes, or just go play fun novel things in QP where nobody cares if they win and overall level of play is much lower so your cheesy tactics might work.
This is based on high-Platinum level gameplay on PC, btw.
/rant