This Tech article about the RX580 Maximum temperature will help you understand the temperatures you are seeing: https://www.onecomputerguy.com/rx-580-max-temp
"Conclusion: What Is The Rx 580 Max Temp?
You should be sure to keep your RX 580 between 70c – 80c for much of its lifespan. You can occasionally push it to 90c, but you don’t want to keep it there. Remember that the lower you can keep the temperature, the better it will perform."