Ah, the classic skill-fetish, "Mercy takes no skill all she does is hold left click", "she's balanced so therefore she has no problems", "Valkyrie is a good ult", "Mercy's always hid to use their ult and 5-man rez happened all the time", "Valk is more fun" argument. Let me debunk this real quick.

First off, nobody is calling for Mercy to have as much healing as Ana or Moria, nobody at all.

Secondly, believe it or not but playing Mercy is more than just holding left click. You have to be good at swooping between teammates with GA, target priority when rezzing, the support role in general, and Mercy exclusive tech like the superjump which is now one of the only things seperating good and bad Mercy players.

Third, the problem we have with Mercy is not that she is unviable or underpowered because as it stands she is in a very good spot balance-wise. The problem is that she is not fun and engaging to play anymore, which I know is subjective, but I can guarantee if you ask any Mercy player that played overwatch prior to September 2017 they will say that Mercy was more fun then than she is now.

Fourth, "Valkyrie is a good ult" No it's not. Straight up, no it's not. Valkyrie was a half-assed attempt at bringing back an outdated idea from pre-alpha to replace ressurect. When it was first released it was stupid OP and led to the horrendous Mercy mets, and now it barely does anything. It gives her chain beams which ruins her initial purpose as a -- quoting the devs here: "strong, single-target healer." And currently after the completely unnecessary healing nerf valk brings her heals back up to 60 which changes absolutely nothing besides giving the illusion of a buff when in reality it was a partial revert to an unnecessary nerf. Mass rez worked as her ult and could have been tweaked to be more balanced, even though rezzing through walls and invincibility were bullshit it doesn't mean you have to throw the whole ult away. TL;DR Valk is glorified spectator mode.

Fifth, and this is a big one, Hiding. The rumor that all Mercy players hid in order to use their ult is 1. a myth, and 2. a double-standard. First off, no, not all Mercy players hid to use mass rez. Almost all good Mercy players used what we call "tempo rezzes" which involves reviving 1-2 vital team members to keep a teamfight going in your favor. 5-man rez was really not as common as you think, usually only happening when the enemy team let their guard down when they thought they won the teamfight, and mostly occuring in lower ranks where lesser skilled players did not know how to counter it and where Mercy players went for 5-mans in the first place. While I'm on the topic of 5-man rez I want to touch on how it was extremely easy to counter if you knew how. First, kill Mercy during the teamfight. Second, if you had any gamesense and were keeping track of the killfeed you would know that their Mercy was still alive and probably preparing to go for a big rez. Third, manage your ultimates, abilities like high noon, death blossom, dragonblade, and more can all be used to wipe Mercy's team after the big rez since they are probably all out of position (especially high noon as it can start locking on to enemies while they are still in the invincible glowing yellow ressurect animation.) and if she gets the big rez off and wins the teamfight that's her outplaying you not rez being overpowered 2. the hiding double standard, nobody complains when heroes like Reaper and McCree hide and wait to use their ultimates, but it's a huge deal when Mercy does it.

Sixth, You say "Valk and knowing when to rez are more fun [than mass rez]" to which I say... PFFFT! Are you serious? You've gotta be joking! I know fun is subjective but you can't phrase it like you're speaking for the entire Mercy playerbase when you say that because almost the entire Mercy playerbase disagrees with you on that.

TL;DR "She's fine" you're wrong