I get what you mean, but I think their stiffness comes from over-preparation. If they could just remember it's just Coachella, not Inkigayo or Music Bank, they would have had more fun with it and let loose, I think.
But with that said, they were able to enjoy it more as time went on during the performance, so that's a great thing! They did well considering all that.
It's just a matter of mindset. They have what it takes in skills, talent, and training. Nervousness isn't too much to worry about; it's something they'll get over with more experience.