Mantis is not a woman full stop. she is an alien side character in a weird alien fantasy movie who happened to share the best scenes in the movie with drax (imo)
How is Gamora not a respected, self confident woman? she is one of the best assassins in the frikin universe, daughter of the biggest badass of the universe and hell bent in revenge. like seriously what movies did you even watch?
Mantis simply put is a side character, she didn't need a massive back story as the film was focuses on other things. You can dislike that all you want but to use it as an example of sexism (I'm assuming you are) is ridiculous to put it lightly.
the way I formulate is the same way most women formulate, people weren't complaining about her character when it was released, you seem to be the first I've seen of this so don't assume all women think the same way you do and don't try to insinuate my line of thinking is the reason women have 'difficulties'. it's a movie, and Mantis was a good edition for a side character, don't overthink it for the sake of getting offended