I completely agree with you, her behaviour disgusts me. Her willingness to believe rumors with no basis and form further form her own misconceptions is annoying and not cute at all.
** spoiler** up to the point where he saves her from the princess who tried to curse her, she still continues to believe he loves the very clearly deranged princess. Her behaviour can't even be said to be plain naivety. She is just stupid and only sees what she wants to believe. The ML honestly could do better.

the fl believing rumors and the princess' one sided feeling bc "they look good together", never asking sergio about it, assuming that he feel nothing for her bc she's self conscious isn't cute.
sometimes humility, naivety and self-consciousness becomes downright malice and she's on the path to that.
what's next, her thinking that he slept with her bc of the mood or bc he's kind? if i was sergio and knew what she thought of me, I'd take it as an insult