It is too low, they need to have room to pull themselves out of the old exoskeleton. At the point where the mantis is nearly free from the old exoskeleton when molting, it will be twice the normal height of the nymph.
What happens is the nymph will become stuck, and unable to finish molting. At that point there is nothing you can do to help, I've tried many times - if it does get stuck it sadly will die.
If it is molting you may try to very carefully move the twig up higher; however be extremely careful because if it falls during a molt it will become stuck in the old exoskeleton or other damage.