Pumpkin pie is a food item, but it has strong cultural ties to England, Canada, and America. Pumpkin pie is made from pumpkin which is a general symbol of the harvest season. Thanksgiving is a well-known type of harvest holiday. And Thanksgiving, as it is presented in Animal Crossing, looks very American (so does Halloween, Christmas, and Easter). So, if we were to get a pumpkin pie food item as a decoration, it might come with Franklin for Thanksgiving.
As a side note, for a game developed in Japan, I have no idea why the holidays look so traditionally American/English/Western instead of having holidays that were more centered around Japanese culture.