This story contains a number of interesting folklore elements. Among them are: transformations of character and/or transformation of physical appearance—Mrs. Bullfrog and to a limited extent Mr. Bullfrog; taboo—Mr. Bullfrog must not object to his bride's grotesque natural looks [however temporary] and must not touch her curls, or he will regret it; a talismanic object—money from Mrs. Bullfrog and connubial love (which can work miracles) from Mr. Bullfrog; mysteries—how did Mrs. Bullfrog happen to disappear after the coach overturned, who had taken her place ("Old Nick"?), where did she return from? But essentially "Mrs. Bullfrog" is yet one more of Hawthorne's "marriage tales" and thus the themes here have to do with the marriage relation.
Among the more significant themes here under the "marriage relation" heading are the following: "The importance of making an effort to preserve a marriage despite uncertainties and serious difficulties"; "The misguided search for perfection in one's spouse"; "The misleading nature of a marriage partner's outward appearance"; "The importance of maintaining a flexible attitude toward important matters"; "The principle that there's no accounting for taste, in others' choices of marriage partners"; "Marriage involves making sacrifices, keeping secrets, and anticipating sizable benefits"; "The fundamental thing that applies in marriage: the match must be a good one, at the outset." Enough...
(The entire section is 241 words.)