It doesn't matter if the charger is rated too low. The device can't draw more than the charger is designed to output and unless the charger design is faulty, it is designed to handle the heat load generated at the maximum output.
Now on some devices if that rated output is too low, the device will refuse to charge at all. Mostly it just means that the device will charge slower than it would with a higher output charger that the device could take advantage of, that or the device drain might be more than what the charger can provide.
For example, the iPad 1 and 2 both will charge on .5amp USB chargers/computer USB ports if in standby/off. If on, the screen and background operation of the CPU, memory subsystem, etc draw more power than .5amp charger can provide generally, so you'll slowly drain the battery is using the device while plugged in to a .5amp charger, but it will still provide the 2.5w/.5amp while plugged in.
Some really high power consumption devices, like the new iPad won't even charge while on with a 1amp charger, it'll just slow discharge and even on a full 2.1amp charger, if using it full tilt, it will drain the battery slightly (at idle/stand-by/off it'll charge it).