I've used a cotton bag liner for winter camping for the past couple years, I love it. I'm usually a huge opponent to wearing cotton in winter, but that's because most people are pretty careless about moisture control these days. If you read my answer to, "Does Cotton Really Kill?", I point out that cotton was worn on the very first summit of Everest.
The important thing is to be dry when you get into bed and properly layer your bag. See my answer to this question about layering in your sleep. I think you're fine with cotton liners as long as you don't over-layer inside your bag. Too many clothes layers will make you feel clammy in your bag, but fewer layers means your bag can get warmer, and having a warmer bag means the moisture inside it is more likely to vaporize and breath out of the bag.
Although it may seem like thin bag liners may add temperature to the bags (and I'm one of the one's who's convinced that they do) they technically aren't designed to make your bag warmer, they're designed to keep it cleaner and more comfortable, a fleece liner would add the most warmth, and it also happens to be the cheapest option compared to silk and cotton. The advantage to silk is that it's by far the lightest and most compact option, but costs almost as much as a new affordable sleeping bag.
One thing that will keep you significantly warmer while sleeping on the ground, is to use a reflective ground sheet, like an emergency blanket, or a nice insulated air mattress. You lose a lot of warmth into the ground due to conduction, so it's important to put something between you and the cold Earth.
Other ways to keep warm at night include eating a lot of calories just before bed (stoking the oven) and making sure you pee before crawling into your sleeping bag, as extra water takes extra energy to keep warm.