She doesn’t need to be the best healer in the game and that premise is outdated, she has the highest maneuverability and survivability of any support and a near game breaking basic ability on E. Ressurrect is Mercy’s burst healing, she brings an average 500hp tank back to life in 1.75 seconds - which is 285hps.
If Mercy needs anything, then she needs Damage Boost to give her ultimate charge for Valkyrie so that she has ultimate more often and by extension more average healing per game. That said Mercy is in a good spot both pick and win rate wise regardless of her average healing, so I believe most of these arguments are disengenuous attempts by bad players to over power their main based on a premise that has no actual barring on the health of the game.