I'm amazed.
Last time I heard, Whisper only ever showed up in comics, she didn't had a prominent role in any of the games as far as I know. The fact that there's a official plushie of her just goes to show how much of a fan favourite she is.
She looks genuely cute, I bet someone will really love having one.

Sometimes, All you need is a cute face and a neat Character design to get a following.