I have a Maple mini sitting on the bench doing nothing, and its reading 28 deg, and my ambient temperature in the room is 23 deg,
ie. its 5 deg above ambient
... and all mine is doing is blinking the LED once a second.
Yours is 8 degrees above ambient and its doing a whole load of other stuff, so its not surprising its 3 degrees hotter than mine in comparable conditions
Look at the spec on the processor
Its well within its operating range.
If I run the board directly with the MPU on the bench, I get over 29 deg
You would probably have been better off posting to the STM32 / mini maple thread.