You could install a fan to reduce temperature. In general it’s hard to find out what could cause it. Usually it’s CPU usage. You could have a look for running process using htop.
DietPi themselves did not offer any notification system outside the DietPi device.
For Raspberry Pi 3 Model B+, the PCB technology has been changed to provide better heat dissipation and increased thermal mass. In addition, a soft temperature limit has been introduced, with the goal of maximising the time for which a device can “sprint” before reaching the hard limit at 85°C. When the soft limit is reached, the clock speed is reduced from 1.4GHz to 1.2GHz, and the operating voltage is reduced slightly. This reduces the rate of temperature increase: we trade a short period at 1.4GHz for a longer period at 1.2GHz. By default, the soft limit is 60°C, and this can be changed via the temp_soft_limit setting in config.txt.
dietpi-banner is giving hard coded information regarding colour code and comments. They are displayed according CPU temperature. I guess there is no real deviation if this is fine on this device or not. Best to my knowledge it’s quite generic. Maybe I’m wrong.