Yes, virtually every single bishop, and abbots, were from nobility. At that time one did not 'work their way up'. You entered the clergy as bishop, often as second or third son in the noble family you come from. There are some records of abbeys being opened in the Aquitaine in the 11-12th century which allowed peasants to join orders as monks, and I suppose in theory it was possible to become an abbot of perhaps a daughter house at some point - but that is speculation. But the records are conclusive that bishoprics were filled with nobility, often an extension of family financial and power interests. Reform of this was one of the chief targets of Gregorian reform, known as the investiture controversy.