By virtue of the fact that she’s lived in isolation for so long and has never known any of her own kind, she has something of an identity crisis as she doesn’t really know who she is or how to act around other people.
She doesn’t really have a problem with the idea either because she has no reason to believe being ugly is a problem.
So, when Drax tells her she’s physically hideous, she believes him because she has no reason not to.
Mantis is one of the new characters that joins Guardians of the Galaxy Vol. 2 as an assistant of sorts to Ego, played by Kurt Russell.