I agree with what Faisal says, you cannot punish a Father for his sons crimes. Islamically we should not speak ill of the dead either.
I also have seen and met many people that do think Yazid and Muawiyah were good people and use the term Hazrat with them. I disagree, Muawiyah was a good ruler which I accept, but he was cunning in the fact that he destroyed the Khalifat and its system of Shura and consultation and replaced it with a Monarchy. He was also not highly regarded by the Prophet (sm.)
On another point, many people curse Yazid and his family and assume that nothing ever happend to the Banu Ummayah, another untruth, when the Abbasids came to power they massacred the Banu Ummayah almost to the last person. From women and children to old man.