As Bloomberg found out together with ifixit, the small HomePod mini has a built-in temperature and humidity sensor.
The small sensor sits near the power cable at the bottom of the case so that the measurement is not affected by the heat that the device itself produces during operation.
Currently, however, the sensor is not feature. Perhaps Apple has installed a bit more hardware than currently advertised and will unlock the two functions that the sensor can provide later with a software update.
This would at least provide a way to measure temperature and humidity in the room where the HomePod is located without additional devices and to evaluate them via HomeKit.