This is mostly something that happens at the highest ranks of play on PC, specifically. Controller input on consoles makes Ana's viability questionable, but that's not a concern on PC. At Master/Grandmaster/Top500, people expect you to be able to play high skill cap characters like Ana in place of Mercy, because she just has a more all-around useful kit. On top of being able to sleep dart things, deny the enemy team's healing, and turning one of her teammates into a nearly unstoppable juggernaut, she doesn't require her team to fight 5v6 while she hides around a corner.Hmm, I've never noticed any hate or harassment directed towards me when I play Mercy (my highest played character still). Is it based on performance or just being unlucky enough to end up in a specific party?
This isn't a great season to be playing Ana (healing with her this season is much more difficult than previously, due to the prominence of D.Va's DM and Winston's bubbles, on top of her nerfs), but even then, Zenyatta is preferable to Mercy in almost all situations. Lucio and Zenyatta are the "meta" supports right now.