I know that this is an old topic but my college fishing buddy made the same complaint.
Either ice them immediately after catching or after you pull them out of the livewell/off of the stringer. I let them sit on ice for a couple of hours. If I’m not in a big rush, I keep them on ice overnight. The flesh will firm up and be much easier to fillet. After rinsing them they go back into at least a fridge. If I am going to cook them later that evening, I spread them out on a baking sheet on paper towels to dry, in the fridge. If I am going to fry them, a quick dip in beer, making sure to drip most of it off, and then into the breading. I set it aside after breading for a few minutes before frying. Then into the oil. Never have an issue with mushy crappie.
Same process if I am going to blacken them and they come out firm, even after being in the freezer for a bit.
I think that the biggest issue with mushy fish is not necessarily icing them, though that is standard practice for me, it is more about letting them dry out as much as possible. Any excess moisture to get the batter to adhere or in the fillet itself and the fish will steam rather than bake/fry. I tend to use several paper towels to make sure that the fillets are as dry as possible.