The term "Mother of the believers" means that no Muslim is allowed to marry any of the wives of the Prophet after him. Every wife of the Prophet is Ummul Mo'minen, but not Ummul Mo'minaat because there is no question of marriage with Mo'minaat.
In Quran (33:53), it very clear that no Muslim man can be allowed to marry any of the wives of the Prophet after him.
Aiysha, was called by a Muslim woman: O mother. Aiyasha replied: I am not a mother to you women, but mother to your men. This is mentioned in Sunan Al-Bayhaqi (7:70) and also in Tafseer Al-Qurtobi (14:123) and also in Al-Tabaqaat al-Kubra by Ibn Sa'd (8:67).
Khadijah (AS) is the greatest wife of the Prophet Muhammad (SAWA), but there were among the wives majority of pious and obedient wives, and few of disobedient wives who went against the orders of the Prophet (SAWA). Even one wife of the Prophet ( Qateelah or Qutaylah Bint Qays) left Islam after him, and got married with Ekrimah Ibn Abi Jahl. (Ibn Katheer narrating from Al-Haakim Al-Neesabori).
No doubt, that marriage was completely wrong, but it proves that not every wife of the Prophet was pious. So, the title of Ummul Mo'mineen establish a rule banning the marriage with any of the wives, but it does not means high status of piousness.
Most of the wives of the Prophet Muhammad were pious and very pious like lady Khadijah, Umm Salamah, Mariyah, Zainab, Jowairiyah, Safiyyah etc.