A thermocline is usually defined as a significant temp change in a relatively small depth change. As a scuba diver I have seen a surface temp of 70 with a gradual decrease to 65 degrees at 18-20 feet, then a rapid drop of 10 to 12 degrees in the next 2 or 3 feet. At 22 feet it might be 54-55 degrees and gradually decrease to 51 or 52 degrees at 35 to 40 feet. In a deeper lake it is not uncommon to have a second thermocline at about 40 feet and a temp in the low 40s below the second thermocline. Sometimes a diver will even see the thermocline as a distortion similar to a cooking oil mixed with water in a pan on the stove. You definitely feel the temp change on your face below the second thermocline. I have to imagine it is like “Bobbing for Budweisers in the beer cooler” toward the end of the party.
In the winter there is not a significant enough temperature variation to have a true thermocline, but even a small temp change may be enough to attract fish. Keep in mind that water under ice will run low on oxygen later in the winter and some species of fish have a tendency to be shallower. Use your electronics and lake maps to develop a game plan. Hope this helps.