If you recently had a sinking feeling that your efforts as Mercy were just not appreciated enough by the rating system, I'm afraid your concerns were entirely justified. According to Jeff Kaplan there was an issue causing Mercy's contributions to have a much lower effect on her Skill Rating reward at the end of a competitive match. This bug only applied to games you won, however, so unless you played only Mercy for the past few weeks chances are it didn't cause you much harm.
Either way, you'll be glad to hear that the bug has now been fixed, and that the developers are still investigating to make sure the whole thing is now well and truly gone. Given how the entire complaint-train suddenly stopped I have a feeling they have managed to do so successfully.
What's really strange is how this issue keeps reappearing with each Competitive season. If memory serves me right this is the third time Blizzard has had to correct healer gains/losses due to an issue with the performance system. Makes you wonder why they haven't just gone with +/- 25 rating like Dota 2 and called it a day. Then again, I suppose they need some way of pushing smurfs to their proper rating as soon as possible, so I think this is a battle Blizzard just can't win.
As a final note, here's the full quote from Jeff Kaplan:
"We fixed an issue that was causing Mercy's contributions to be lower as it pertained to Skill Rating reward at the end of a competitive win. We're still investigating though to make sure the issue is entirely fixed. The fix we put live was integrated on Tuesday but we're still hearing some reports. We'll keep working to fix it if it in fact is still broken. We're trying to verify right now."