Mount St. Smurf is a volcanic mountain that erupted in the cartoon show episode "The Sky Is Smurfing! The Sky Is Smurfing!". The volcano produced ashes that were responsible for Papa Smurf's allergy and had last erupted 300 years prior to the present time of Season 2.
The name of the mountain is most likely based on Mount St. Helens, which had erupted around the time the episode was developed.