I really can't see any evidence that Dora's parents were hated in any way shape or form. I mean, they're certainly unconventional, but this is a comic where the other parents involved include a professional dominatrix, a gay nightclub owner, the creator of AI and a woman who wouldn't be out of place as the villain in a Bond film.
But the Bianchis'? I can't see anyone hating them. Sure, they might have favoured Sven over Dora or told Marten stories about Dora that really embarrassed her, but at the same time, Dora's mother only appeared in that one storyline with her father and her father had a close encounter of the Faye kind in a brief appearance.
Nothing really says that the forum hates them, absence of proof is not proof of absence and that applies here too. Just because no one actually picked that option, its not an automatic universal revelation that they are despised. Its just weird thing to come to a conclusion about.