The traditional Mirepoix would rely on natural sugars in the carrots and onions. Its flavors are intended to provide a background for the primary ingredients in the dish. I believe the onion aroma and herbal influence of the celery would be lost in the excessive sugar in onion jam and celery sorbet and taking the trouble to prepare a sorbet just as an ingredient in an entirely different dish doesn't make sense to me. Once the ice melts, it's only contribution to the dish is more water. If your intention is to plate them as individual items as a dessert presentation I don't believe the term "Mirepoix" would be appropriate.