Mountain is a natural sign, and you have two ASL options. First, you can sign mountain like hill: take both flat, open hands and run them up an angle in unison, indicating the slope of a mountain. Otherwise, you can start both hands as fists side by side, then open and stretch them up at an angle in front of you, indicating the slope of a mountain with rocks or boulders.
Use the sign for mountain when you are out taking a nice countryside walk or a road trip with your baby. It's also a good opportunity to teach your baby the other signs pertaining to nature, or the shapes and sizes of land formations and bodies of water.
Teach your toddler this sign in conjunction with other land formation and water form flash cards. However, teaching them such signs in front of the real thing is priceless.