According to her YouTube page, she's 17. According to your profile, you're 23.
If those ages are correct, it probably has to do with the fact that for you, sexual relationships with anyone under 18 can be considered illegal. Even if you and her are just dating, I'm sure this would be at least part of her mother's concern.
The age difference isn't really that much in the big scheme of things, but opinions about that are kind of irrelevant. Right now probably isn't the best time to be pursuing a relationship with this girl. You could be in some major trouble if you try to keep your relationship behind her mom's back and she finds out about it. Try to stay in contact as friends, but wait a couple years before expecting anything else.