Compared to what? And in what way?

Physically - probably easier than road crew worker or coal miner or ditch digger or an olympic rower. But if you are an active person - harder to sit there and not move much.

Mentally - it is certainly not rocket science and there are not strings of things to memorize or anything really that must be done. But they do have to keep their own boundaries and are not supposed to interfere with what others do even when they have to watch them do something the therapist would not choose. They don't get to really do anything. They have to appear interested - that can get exhausting.

Emotionally - I say it is easy. The problems they see are not theirs to fix. All they have to do is stay back. They blame and label clients even when it is their own mistake and when pressed they call it an art so no one can pin them down on anything.

Personality - being a therapist is never a job I thought sounded fun or interesting - so it would not be an easy job for me.
If one is voyeuristic, likes to play around with people's emotions and affections, and finds the never ending description and depth of human suffering a playground - then probably easy.
And so forth.