My 2017 MacBook Pro 15" gets really hot when it's connected to an external monitor. The temperature reaches 65 Celcius, and I am wondering if this is considered an okay temperature? If it is not what is the solution for such a problem?
If you're talking about the CPU temperature, then yes 65°C is okay.
The 2017 MacBook Pro 15" model was available with three different CPU's with processor speeds of 2.8, 2.9, or 3.1 GHz.
All three are rated to 100°C for the maximum temperature allowed at the processor die.