They are all hating on her because she's family of apparently a guy who let his people starve to death, which made him so hated and caused his downfall. Logically, because this comes from Doflamingo's mouth, you need to place some serious question marks on what now truly happened. Still if there wasn't anything wrong in the country, there is no way Doflamingo could have taken it down with the help of its own people. How does that rhyme with the daughter and granddaughter of that king living in a normal house selling flowers? Considering they knew where they lived, people should have known they were so 'modest'. Still they hate her for it?Looks like we got a mix up of Vivi and Hody jones' situations here. Rebecca with all the similarities with Vivi and also like hody was revealed to be the killer of Otohime in last,, its possible that at the end of this arc, the people of Dressrosa will get to know that Doflamingo(maybe) was behind the attack.
Though, I now hate the people of Dressrosa.U_U
Modesty much?
Yes probably Doflamingo had some elaborate scheme to frame them, but still.