In theory the diode does not have to be de-rated.
The reason why is because its forward voltage drop is constant.
In reality, there is an additional resistance component, which is also temperature dependent.
For example, the 100 amp reverse blocking diode of a transistor i have dozens of:
1 amp 0.56v at 125C, 0.7 volts at 25C
10 amps .75v at 125C, 0.9 volts at 25C
70 amps 1.15 volts at both 125 C and 25C
100 amps 1.3 volts at 125C and 1.25v at 25C
for that semikron diode block, at 30 amps each diode is going to drop 1.1 volts at 150C and 1.15 volts at 25C
this is a pretty typical diode "rating"
so assuming a resistive load, 30 amps rms ac going into the diode block is going to dump 66 watts into the diodes, which means the diodes are going to be 46C warmer than the case is. so if the case is held at 75C, that is totally reasonable and the diodes are going to be running at 120C.
If the current however is not a sine wave and you're dumping the current through the diodes in 50 amp pulses, then the diode voltage drop is going to increase, to, i'm guessing 1.4 volts. so keeping the average current at 30 amps that means 84 watts instead.