Portrayed by Pom Klementieff, the cinematic iteration of Mantis is pretty faithful to the comic book version of the character, before she traveled into space following the wedding of Scarlet Witch and Vision in 1975. Klementieff's pale skin and antennae do justice to the creation of writer Steve Englehart and artist Don Heck, although the former wasn't entirely happy with the changes made for the big screen.
However, the MCU almost featured an even more bug-like version of Mantis, complete with green mottled skin that would have emphasized the character's insectoid name. In the case of her eyes, they'd be more human-looking (like her original comic counterpart), rather than the big black orbs we see in the movie.
Early concept art was posted by Andy Park, Marvel Studios Director of Visual Development & Concept Artist, which you can check out below:
Mantis did have green skin in the comics, but not until later.
The niece of a Vietnamese crime lord, Mantis was raised by a sect of the Kree that trained her to fight and taught her the ways of the Cotati, a race of self-aware plant people; her bond with the Cotati resulted in her empathic abilities that allowed her to sense and influence the emotions of others. She was given the name "Mantis" due to her skill in besting male adversaries.
Mantis wouldn't gain her greenish skin until she melded with a Cotati that she married.
After joining the Avengers, she began a romantic affair with Vision, much to the anger of Scarlet Witch. When Wanda Maximoff finally did wed Vision in a ceremony in Vietnam, Mantis left the Earth as pure energy, traversing the universe in a search for meaning and eventually joining up with the Guardians of the Galaxy.
Along with the rest of the Guardians of the Galaxy cast, Klementieff stood by writer/director James Gunn after he was fired by Disney. Not only did she sign the open letter urging Disney to reconsider their decision, she also filmed a short video of her writing: "We are Groot. We are a family. We stand together."