Chalk it up to James Gunn having to put his own spin on things--as legendary comic book creator Steve Englehart is not happy with the way Mantis was featured in the Guardians of the Galaxy 2 movie.
Mantis and Drax were basically made out to be buffoons in Guardians of the Galaxy 2, and in an interview with Polygon, Steve Englehart, who created Mantis back in 1973, offers up he wasn't too thrilled.
Well, I was not happy with Mantis’ portrayal. That character has nothing to do with Mantis. I will say that I liked the film quite a bit overall, they’re doing good stuff and I enjoyed my night at the movies so long as I turned my brain off to the fact that that’s not Mantis up there. I really don’t know why you would take a character who is as distinctive as Mantis is and do a completely different character and still call her Mantis. That I do not know.
In the comics, Mantis is actually a human woman, but for some reason, James Gunn only wants Star-Lord to be the human "relatable" character in his movies, which is why he cut Richard Rider Nova from the first movie's script, and we can also guess why he made Drax into an alien, Mantis into an alien, and to an extent Ayesha and Adam Warlock into aliens.
Steven Englehart did go on to note that he did like the character of Mantis in Guardians of the Galaxy 2, but again stated it's not the comic book version.
“I wasn’t impressed with what they did with Mantis but the Mantis on the screen was entertaining, I liked her but that’s not Mantis.”