It's up to you, how close you two are if you want to call her 'babe' back. Most girls just say it offhandedly. It also might be an indication of interest, but you'll have to put the rest of her behaviour into it to figure that out.
The appropriate response is to be cool, calm, and NOT to reciprocate, but to just take it as a mild compliment. In and of itself it means little, but it indicates friendship and/or admiration and affection. It's a term of endearment.