It depends on the ambient temperature (the temperature of the air the phone is in) and the amount of motion of the air. A phone in an 80 degree wind would probably run cooler than a phone in 75 degree still air.
On a really hot summer day, when the air is 100F (38C), the battery can't be lower than that, so a battery that's 38C is like one that's not even in the phone on a really hot day. Since the battery generates SOME heat, 40C on a hot summer day would be low. (Out in the open, in the Antarctic in winter, that would probably be a battery that's about to explode.)
We'd need the ambient temperature when you measured that battery temperature, but in any environment that's at least somewhat comfortable to you (IOW, not more than about 110F), 38C isn't high for a battery.