YYA, I agree on severity of those who joke about others’ lineage. At the same time, we should not be too haughty. R Zeira silently rejected R Yohana’s daughter as he thought that yichus in EY in his times was not good enough. Where does this leave those of us who come from Sanzer chassidim!?
To extend your logic, I thiojnk, good middos is the only sure sign of good yihus, and opposite – of opposite.