It's not really that weak, it does 20 damage per shot or 100 damage per second and it does deal head shots as well, so if Mercy is able to use it properly she can kill quite many heroes easily and quickly enough with it. And it only takes 100 damage to destroy that tire, so it is not that hard to kill when you know how to deal damage.
Interesting. Math...
Mercy: 1.4s reload, 20 shot capacity takes 4 seconds to shoot and does 500 damage if it all hits. 500 damage/5.4s = 93 damage per second maximum.
Reinhardt: 75 damage per swing per 0.9s, no reload, 83 damage per second.
The difference is that Reinhardt can hit multiple opponents per swing and it's dead simple to hit, but his range is very limited.
McCree: 6 shots, 2.38 shots per second, 1.5s reload, 4 seconds to empty and reload, 35-70 damage per shot. Maximum 420 damage/4s, or 105 damage per second. Half that at range. Up to 75-150 dps with fan the hammer, if you hit anything
I remember the first DPS I killed with Mercy. Soldier had me trapped with nobody to guardian angel to, so I could just die or I could shoot him...and he somehow lost that duel.