Whether Della and Jim were or were not foolish depends upon your point of view. There are arguments to support either opinion.
Jim and Della had very little money available for things like presents of any sort. They lived in a very inexpensive flat with rundown furnishings and had to watch their expenses very closely, with "pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher." These are not the kinds of conditions in which people should be spending relatively large amounts of money for extravagant gifts. From that standpoint, they were very foolish.
However, Jim and Della loved each other with a depth of feeling that reached the depths and heights of their beings, a love that demanded and deserved a demonstration in keeping with its enormity. Both willingly and gladly made a great personal sacrifice in order to be able to present the other with a gift that adequately expressed the great love they shared. The greatest treasure of all was their shared love, and by sacrificing in order to give gifts to honor and celebrate that relationship, they demonstrated that "of all who give and receive gifts, such as they are wisest."