Bayley could have gotten over big as a babyface. She was given some terrible angles to work and got rather buried.
A heel turn COULD help if they booked her right and made a story with it. But know them, what they will do is turn her heel and then randomly bring her out with no story and just beat up on or job to faces. It will fail.
I think they should turn her heel by making her just a hyper version of what she already is. Imagine her hugging EVERYONE, always talking about how she is doing what she is to make her young fan proud just be as obnoxious as possible about it. She would start getting mega heat.