Diego-san likes to sit on top of the piano because Theo has a difficult time figuring out how to get up there. Long ago he actually liked walking back and forth on the keyboard improvising a tune as he went along.
I'm finding this more and more common that Diego-san tolerates having Theo next to him. I think a lot of times it's just too much work to get up and move but maybe, just maybe world peace is just around the corner.
One of my favorite Sesame Street bits involved Grover trying to teach the concept of near and far. First he would run up close to the camera and explain this was "near." Then he'd run away from the camera to a spot and say this was "far." He ran back and forth until he was exhausted and keeled over. Thompson and Diego-san have a much more sedate way of explaining far and near.
What you can't see in this photo is that the camera guy is mimicking Thompson's pose by lying on the carpet, face close to the ground, arm outstretched. It appears Thompson isn't too fond of being mimicked.