No it needs to regulate the temperature of the heat pad, not the temperature in the enclosure. Placing a thermostat probe in the enclosure would mean you would need to heat the pad hotter than the probe reads to get it to regulate to the set temperature. Placing it on the pad means you set the thermostat to a desired temp and that probe regulates the pad based on the actual temperature not what is inside the enclosure.
You use thermometers, thermometers with probes, or best an infrared temperature gun to gauge the inside heat and you do so from the material in the enclosure, not the top of the substrate.
Thermostat = equipment to control a heating device
Thermometer = device to measure heat from a heating device.
These are important words to get right
Also this is not something debatable.