To be 'po
-faced' is to be excessively self-righteous, stuck-up, or just generally devoid of humour. The Daily Mail
newspaper is constantly po-faced; as were Emerson, Lake and Palmer
The term itself is a reference to the word 'po', an old English term for a chamber pot (in other words, a portable toilet) - the implication being that an insufficiently earthy person will, when presented with a freshly-used chamber pot, pull a face of disgust.
It is not a reference to the famous Teletubby, although both words share the same pronounciation.