Mercy is fine but her offensive isn't as good as other supports like Zen and Ana. Mercy's healing is probably better tho. Also her ult is a game changer in modes like capture points or KOTL. She doesn't have a lot of offensive choices so she has to latch on and rely on the hero you're healing. Guardian Angel is not that good compared to other support hero abilities, but it's still useful in it's own way :P
Mercy's healing is better than theirs by a long shot but Zen's ult is usually better than Mercy's and always more reliable, and that offensive can't be ignored. Ana's offensive is honestly mediocre imo, what's really her selling point is her Sleep Dart (+ she has great range + she has Nano Boost every 8 seconds + healing buff works well with Lúcio/Mercy). Lúcio is the pref with the lesser-healing supports because of his AoE heal + they're immobile so Speed Boost can help compensate for that a little. I rather have Transcendence in almost all situations than Resurrect, especially on capture points (because that means it's harder for Mercy to hide). I think that right now Mercy's Res is actually the most useful when you're pushing the payload (Transcendence is really nice on chokepoints on those maps and in situations where you're dealing with wipes towards the end, but in open spaces or areas where Mercy can hide to pull off or charge her ult a little Resurrect can be stupid strong just not being greedy and ressing a Rein). Payload is minor AoE heal too which helps her survivability a bit around the objective ime, though you shouldn't be sitting on the payload too much unless your team is really awful.
But the Zen nerf may change things. I think Blizz is also looking into giving Mercy some more more innate mobility since most people don't know how it works and will just put you in awful situations without thinking. I saw a suggestion for giving her the ability to lay a little ball of light (the same that spawns on corpses) herself so she can fly to that. That would be
really strong on some maps and not so strong on others possibly, but what I like about it is mostly some people are just dumb and won't get in your damn line of sight to let you lshift to them. Mercy and Ana both have something awkward about them and that's that they're designed to babysit but they need intense babysitting themselves often, especially Mercy. Lúcio and Zen are much more self-sufficient (even though Zen is immobile and Ana has her Sleep Dart).
Mercy still strong though, just outclassed atm by ridiculously good Zenyatta's ability to bring dps to the team and the fact Transcendence on a viable Zenyatta brings as much or more to the table. Also McCree being nuts right now is awful for her, even though Widow is gone, and Genji and Winston being very common (Mercy has a way to run away from Winston at least, but you better hope it's a long way away). Zen is thriving with Widow gone, Mercy not so much.
Although speaking of meta factors, Mercy + Zarya is dope as fuck. I love picking Mercy when we have a good Zarya.