There is no cause for concern or worry at all. Clobetasol in orabase is in too small in quantity and is very little absorbed from the oral cavity. Even otherwise clobetasol is not contraindicated in pregnancy.
Category C only means that on theoretical basis (mechanism of action etc.) certain drugs may cause harmful effects on the foetus without causing malformations. Such effects may be reversible. Clinical trials to see the effect of such drugs in humans can not be conducted due to ethical problems; hence to meet regulatory requirements and avoid legal actions such drugs are routinely out in category C.