A candle typically puts out around 250 BTU. Not enough to do anything useful. Maybe a nice psychological comfort, but as far as actual heat, it's like trying to heat your vehicle with a cigarette lighter.
And no, putting a flowerpot over it won't improve anything.
In a sleeping bag, one trick used by backpackers is to put a large plastic trash bag inside the bag. This will trap heat. Alas, it will also trap moisture, so you will be wet in the morning. It's for emergency uses only, for one night.