Maybe. Mercy has different tools and uses that Moira doesn't have. Res in general is an incredible ability in a game and Moira's heals have a meter and a cooldown for the orb. Mercy's heals don't have to be aimed and never run out plus damage boost and being able to do both from behind cover.
I find Moira easier to play with no team support. More often then not when I play I end up going healer and nobody looks out for you, mercy is fairly squishy and I found Moira can get out of a fix easier
All down to playstyle. For me, Mercy will always be the hero whose play suits me best,, but it's natural for others to gravitate towards other healers.
And for every person that doesn't play Mercy, that just makes me more likely to get her <3
Moira doesnt quite get out as much healing as Mercy but she does more than ana, zen and lucio. She gets more elims than Zen (Zen does more damage though) and she has a great escape. She can solo heal, you just need to keep the balance between damage and heals. She's a great hero and she is 100% worth learning and playing. Also some stats to prove what im saying.