Symmetra isnt bad, she just one of the worst offense heroes cuz her turrets and portals are usually out of place when the payload or cap moves, and if shes the only support then the healing aint gonna be there for the team like Mercy or Lucio. Requires a lot of map knowledge on where to play sentries/portals which I dont think the average player has right now. Most would be better off picking Lucio.
If you suck, just pick Lucio and stand around with ya squad mashing the E button. You're so fast nobody can kill you, you can escape any situation with speed up/e button, you pump out good damage with your left click that can legit kill people, you get people to the point faster, you heal them all by standing around, you have the most hype Ult in the game (*jumps in the air, music builds up*"LETS BREAK IT DOWN!" *echoes and reverberates waves of awesomeness) and its effective like all the time. He's the best support.