“I don’t understand why it’s a problem for the boy’s mother to want the best possible girl for her son. How is that unreasonable?”
Most mothers probably are solely interested in the best interests of their son.
However there are some mothers who probably envision having a certain type of girl as a daughter in law and will be biased towards that type.
For example if a mother and her daughters tend to dress “trendy” the mother might be uncomfortable or embarassed to have a daughter in law who dresses out of style. She might be self conscious to take her dil to shul with her etc..
Her son might not care at all whether his wife will wear the latest styles or not.