There are a few reasons why the "Mantis is ugly" dialogue and plot line does not work. First of all, it relies on the archaic and problematic idea that if a boy is mean to you, he likes you. It's clear throughout the film, that there is a romance brewing between Drax and Mantis, fueled primarily by his animosity and rudeness towards her. The more he reminds her that she is ugly, the more she starts to trust him. "I'm ugly?" she asks him. "Yes! Terrifying!" he says, looking her up and down while making a horrified face. (I'm paraphrasing.) This happens more than once, and since these are her first ever friends, and potential allies, Mantis hangs on to every word the group says to her. "I'm ugly!" she repeats to herself at one point, gleefully because she has no concept of the insults being hurled at her. Because of her character's secluded upbringing, she's like a little kid. So just imagine children watching this movie, and hearing a strong male character telling a sensitive and empathetic female character that she is unattractive until she is conditioned to think so herself. It's awful. And that scenario coming to life was all I could think during those scenes.