"Motherboard temp" is a combination of things, and it might be different from board to board and manufacturer to manufacturer. Even the utility you use may vary, or even be wrong. HWmonitor is often guilty of misreporting some specific sensor data due to identifying sensors incorrectly.
HWinfo is the utility you SHOULD be using, in most cases. Ryzen master as well, for AMD platforms.
Generally, "motherboard temp" is a calculation of sensor data taken from several different components on the motherboard and averaged to represent motherboard temp. It will react MUCH slower than other "sensors", such as the CPU temp. If you want to know specifically whether a specific component like the VRMs are overheating, then look for sensor data labeled as such. It won't always be there or might be called something different, but generally if it says "motherboard temp" it is not the data from a single source. And, that is generally the best "sensor" value to use when setting up case fan profiles if you don't want to get driven crazy like you would if you assigned the CPU or Core temp sensor data to drive them. Obviously, for the CPU cooler you want to use the CPU core temp data though.