Did Mrs. Mallard die of joy?

No, the major irony of the story comes in the surprise twist of fate at the climatic end. Louise thought her freedom would come from her husband's death. Instead, she gains freedom from his domination only in her own death. The joyful, long life she imagined lasted for just an hour. However, in that short time she experienced how it felt to be an independent person, unfettered by the constraints of societal oppression. Even more ironic is that the doctors surmised she died because she was overcome with joy at seeing her husband alive. Nothing could be further from the truth. She died upon learning that her new freedom had ended before it could really begin.