This was always true of Symmetra Glad you're enjoying it as much as I doI'm having a blast on Symmetra in QM. I'm giving no shits and play extremely aggressively with her. Basically flank and sneak behind the team, and then going straight for the supports. It works so well in QM.
Many hilarious moments where I am just cracking myself up, like hiding in the Eichenwalde tower waiting for a Zenyatta to walk by and jumping down like a bat from hell just to see them turn around in surprise while dying. Then their teammate turns around to see what happened only to die as well. So much fun.
Honestly it is too easy sometimes. I'm not sure if there even is any mmr in QM, it just seems very random and average to me. It's gotten to the point where I'll leave defense maps trying to get an attack map for more of a challenge. It's just so funny to me to be the first person as Symmetra to manage to push through Hanamura or Anubis gates, especially if that team was just complaining about the Symmetra pick. I have noticed not many people complain about Symmetra anymore though which is awesome.