That’s only because she has received no lore (aside from being a side character in other characters’ stories). Once we see more of her with her own POV and her own story, we’ll be able to understand her better.
Zenyatta, for example, is in the literal same position as she is. He’s a concept, he’s mostly used as background to Genji, he is easily defined by vague descriptors. But people never say he’s boring. They go out of their way to develop him and give him interesting traits and invent potential storylines and conflicts for his lore.
There’s no reason not to do it with Mercy. Sure, maybe someone isn’t personally invested in Mercy so they don’t care. That’s fine. But there’s no point claiming she has no personality then. Otherwise there’s no reason not to give her the standard fandom treatment.
It’s not a datamined voice line, as far as I’m aware, unless there was another one. The interaction is this (it’s still in the game):
Mercy: What happened to you?
Reaper: You tell me, Doc.
People jumped to a conclusion that this meant that Mercy is the one who created him, but that never made any sense to me. He’s just saying “You’re a doctor, you should tell me.” He’s not saying “You’re the one who made me, you tell me.” Why would Mercy ask him in the first place if she’s the one who made him?
It was cleared up because it’s not correct and because the writer obviously knew that it’s an upcoming character (Moira) who is responsible.
This isn’t stepping on the fandom’s right to innovate and be creative, it’s simply making a clarification on something that some people misunderstood.
Even then, fans are still free to have any headcanon they want, including Mercy being responsible for Reaper. It’s not the lead writer’s job to keep all theories within possibility though. Canon is canon, he can’t cancel Moira because some people want to HC Mercy as being responsible for Reaper.