平(píng)凡(fán) is a neutral word with the meaning of ‘ordinary’ while 平(píng)庸(yōng) is a derogatory term with the meaning of ‘mediocre’.
The difference lies in the semantic distinctions of 凡 and 庸. The former means ‘ordinary/common’
while the latter means ‘unwise/mediocre’.
One person can be ordinary, but should not be mediocre.