The issue isn’t that Mercy “is useless”. The issue is that when it comes to supports, just about every other support is a better pick than Mercy - Across all elos, and she needs to be made more viable in the meta.
And If you’re going to use statistics, you should probably mention her pickrates as well and her comparison to how other supports are performing.
Mercy’s winrates look “high” in those areas, because her sample size is a lot smaller than it was before the nerfs.
Her pickrates are 1.62% in GM
2.65% in Masters
and 3.44% in Diamond.
This number climbs steadily up to 8% in Bronze. Currently the elo with the most Mercy usage by the way.
As you see in her win and pickrates, generally the higher her pickrate, the lower her winrate. This has happened to Symettra and Torbjorn as well. Less people playing the hero = more skewed data on how viable they really are. So winrates alone don’t tell the whole story.
If we are to use winrates, and look at the big picture, compared to other support heroes very little has changed. She is still the worst support pick on average across all the elos, with one of the top 5 lowest winrates on average in the game. Beaten only by Roadhog, McCree, Winston, and Sombra. Solidifying her as a niche pick at best, and a troll pick at worst.
Note: Her being niche or a troll pick isn’t a problem by the way. The issue lots of people have with her is the fact that she isn’t engaging or rewarding to play, with very little impact. So playing her kind of pales in comparison to what other supports offer in their kits right now. But anyways, back to the stats!
In Bronze? Mercy is the Second Lowest. With only Ana being lower.
In Silver? Mercy is the Second Lowest. With only Ana being lower.
In Gold? Where Mercy is usually the strongest? She is once again the second lowest of the Supports. With only Ana being lower.
Things only get worse from here…
Platinum? She is the at the bottom of the Supports with the Lowest winrate.
Diamond? She is once again, at the bottom of the Supports with the Lowest winrate.
Masters? Mercy is the Second Lowest. With only Lucio being lower.
GM? A special case. Even with Mercy having the highest winrate than any of her other elo’s, Mercy is still the third lowest support overall in winrate. With only Lucio and Moira being lower.
So in short, is Mercy “useless”? No she isn’t. Is she a viable pick however? Looking at those stats? Debatable. Is she underpowered and in need of some sort of compensating tweaks or buffs? I believe so. As of right now, there is very little going for Mercy that would make you want to pick her outside of personal preference, with the utility creep of other supports going on, you are better off picking Zen or Ana in higher Elos, and Lucio, Moira in the lower elos. Or… Just pick Brigitte since she’s consistently viable in all of them (and is arguably a lot more rewarding and impactful to play than Mercy to boot, in my honest opinion). 
~Sincerely Yours xoxo,
a Lover of True, Fair, and Fun Balance.
xavvypls 