You absolutely should salt the zucchini before adding it into the cornbread casserole mixture. Salting it will pull out some of its moisture. That way, when you cook it, it will be firmer, assuring that your cornbread casserole will hold firm. You should salt zucchini in almost any recipe. If you have sliced zucchini, lay it on paper towels and salt it. After 30 minutes, pat each slice dry with paper towels. Then, grill or cook as usual.