As I understand it, Lopt blood is hated/feared across Jugdral, because people don't want to return to the horrible evil empire days of Emperor Galle. They fear that someone with major Lopt blood would bring back the old tyrannies and child sacrificing with the help of the Lopt Cult (as did happen with Julius later.) Thusly, in every kingdom in Jugdral, there is an ongoing "witch hunt" for remaining bearers of Lopt blood. Diadora escaped persecution because she was brought up in a hidden village, with her descent presumably kept secret from everyone in Verdane, except the other hidden-villagers. The village is where the descendants of Saint Maira and his attendants had lived for centuries after Maira had defected from his predecessor Galle's cause. Because of that, the people in the village understood that Lopt blood alone does not make someone evil, but at the same time, they kept the remaining descendants (Diadora) tightly under control, so the blood would not be spread. Even people associated with the legacy of Maira don't want Lopt blood getting around, because the Lopt Cult is out there to make sure the blood is used for the dark lord to be revived.
Alvis grew up in a completely different enviroment - among the Grandbell nobility. Unlike Diadora, there was not anyone around who would be sympathetic to his situation. He lived constantly with the terror of being found out, knowing he could be denounced and burned at the stake. When the Lopt Cult found out about his lineage and approached him, he had to cooperate, out of fear the cultists would reveal his secret. Hence, he got swept up in the events of FE4 and ended up married to Diadora so Loputous could be revived.
So, TL:DR; response to your question, no one is accepting of Lopt blood, except the crazy Loputous worshippers who want to bring back his power. Diadora just had the luck to be born in a community that was slightly less assholish about it.
Edit: More info about Galle, Maira, and Lopt witch hunts in the designer notes in this post, under the heading "Secret of Blagi Tower."