What you are observing is normal. Most insects have a natural tendency, or an instinct, to go up/climb to the highest point possible. Mantids are benefited by being at a high point when preparing to fly. The higher they are, the greater distance they can fly with less energy used than if they started on the ground - not to mention that they won't accidentaly smack and crash land on high grasses and weeds in their way by starting at the highest point in the area.
Another reason is that mantids are hunters. Whether they lie and wait for their food or chase it depends on the species. Your mantid may be looking for food up there that wouldn't be found on the ground. Mantids often climb plants, such as lavender, to find butterflies and bees.
What are you keeping your mantid in, and how big is the enclosure?
Example: If you've ever held a ladybug, you may have noticed that they climb to the highest point possible on your hand before flying. The same goes with mantids.
