Early general election polls have a high variance, but the central limit theorem always holds. In ordinary terms, those polls aren't individually good at predicting the election outcome, but when a lot of them say the same thing, it means there is a problem. In this case it means Trump would probably beat Clinton if the election were held today.
Sanders may or may not weather Republican criticism better than Clinton; we'll probably never know for sure. But, when you consider how unusually often Clinton makes an unwise decision that leads to a national scandal, I'm thinking she's actually earned a lot of the distrust and paranoia that's sapping her approval rating. Like her private e-mail server for state business might have been an innocent convenience, but it is certainly a stupid thing to have, and she definitely knew it was against the law. It's also a terrible idea to court the bulge bracket in 2016, especially via secret speeches of the sort that ended Romney's political career. Barring any assumption of malfeasance, the nicest thing you can say about Clinton is that she's a well-meaning goofball who accidentally finds herself in a new unethical situation every week, like if Larry David wrote House of Cards, and saying that the Republicans would have an easier time with Sanders is just a little bit of a stretch.