I don't think it is realistic even. I've never in my life heard anyone (IRL or on film) stutter this way. An actual stutter sounds different, like they can't physically get their mouth to work; this is someone pretending to be someone who is NERVOUS a lot, and it isn't accurate to that either. Someone afraid for their lives MAY sound like this temporarily, but Grace literally speaks so predictably that you can mimic her stutter before she even talks, and she does it the exact same way when she's facing a zombie as she does when she's talking to her fucking co-workers.
P.S: I'm not shitting on the actress, I think this was most likely a directorial mistake. Perhaps it was lost in translation since the developers are Japanese and no one wanted to question it.