There's no doubt that characters change when they move from one medium to another. It would be nice if all character were able to remain completely true to their source material, but no one is under any obligation to not take any creative liberties when making an adaptation.
The character of Mantis in Guardians of the Galaxy Vol. 2 was a welcome addition to the team, but she did depart from her comic book origins significantly. Mantis co-creator Steve Englehart recently spoke to Polygon and was not exactly pleased with how a character he created ended up on screen.
"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."
Englehart's statement is a common one that happens when a character is changed significantly from one medium to another. The Mantis of the comics has a long 45-year history. She started out as a human woman who was raised in a Kree cult as the potential mother of the "Celestial Messiah". Mantis received training, but ultimately had her memory wiped before going out into the world and meeting the Avengers.
Mantis would walk the line of villain for a little while before Englehart was able to complete the story line he started in 1973. That is a story that doesn't really work in the current evolution of the Marvel Cinematic Universe, even something as bizarre as the Cosmic side of it, so it's not surprising that she was reworked into a different character.
Despite the fact that the character was not the one he created initially, he was quick to amend that he still enjoyed the character and the movie as a whole.
"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."
Englehart also spoke about the creator compensation for characters and ideas and whether or not Marvel has reached out to him at all over the use of his characters. Mantis is not the only Englehart creation coming to screen this year; he also helped create The Defenders, which is also getting a dramatic retooling from it's original form. Englehart, however, was mostly positive in how Disney and Marvel have treated him.
"I think Marvel's been pretty open-handed about that sort of thing, or I should probably credit Disney for that. They've been good about that kind of thing, so I'm happy when it benefits the creators, not just me for myself but for anybody."