My wife (I stutter, she doesn't) watched the series and is now reading the books. She hasn't mentioned anything, but she knows I usually don't react well whenever stuttering is portrayed in pop culture. I'll have to ask her about it. You pretty much summed up how I feel about it. Characters who stutter are usually flawed, weak, and sniveling (looking at you, Mr. Potter). I haven't watched it, but it sounds like they got the aunt right in that she's that person we've all experienced who claims they stuttered as a child but it went away, or that person who knows some secret technique that helped them get over it and they're eager to share it.