Mantis' story is a bit different in the 'Guardians of the Galaxy' movie than it is in the comics, but isn't that true for pretty much all of the superheroes? Just ask the X-Men. Who even knows what comic book those movies think they are following at this point. In the film, she plays Ego's sidekick/nap helper. She can use empathic powers to make him fall asleep. I guess being an all powerful god isn't great for your sleep.
I'd love to have that power. I'd constantly be touching people to put them in a good mood. You know what that means? Dance party wherever I go. Happy people are always down to dance. Once I got bored of doing that I'd probably make people depressed just for laughs. I know, if I was in a Marvel movie I'd be a villain. No shocker there.