Is the back of the case hot as well?
THe benchmark is ‘too hot to handle’, as in can. you comfortably touch it or does it hurt you when you touch it?
Some heat is normal. the battery when it charges or discharges puts off heat. Or when you play games or videos that stress on the processor can generate heat.
But it should never be more than really warm.
If it is too hot to touch then stop using it, DO NOT CHARGE IT. ANd make an appointment at an apple store or authorized service provider and let them check it out.
Batteries make energy through a chemical reaction, and if something is wrong that reaction can generate more heat than it should.