Even with modern electronics, when your dealing with power (and chargers are handling power) inevitably heat is generated.
I suspect it is normal, and nothing to worry about. All the well designed units should have some sort of thermal fuse or over temperature trip should the charger get hotter than the designer intended.
If you continue to be concerned about it, you would be better referring it to your supplier or the manufacturer who should know more about the specific model of charger, and its thermal properties.