It depends on quite a few things, but could be fine. Neverthless, best to have it checked if you are at all concerned.
If the earth pin (long one at the top) was hot, stop using the thing immediately. It is unsafe.
Electric heaters can draw quite a lot of current. It's the current passing through a resistant material that generates the heat.
If you were running the heater at the highest power for an extended period, it's quite likely that the pins on the plug would heat up – not because of the resistance of the pins. More because of the relatively high resistance of the contact between plug and socket.
I'd certainly try it in a different socket.
I'd also run it at low power for an hour or so and then check the pins for temperature.
Then try running it at a middle power setting for an hour and check the pins.
The pins shuld not get hot after either of those. If they do, then get the thing checked and probably best not to use it in the interim.
If the pins get only warm, then try running it at full power for five minutes and check again. repeat for intervals of 5 or 10 minutes.
My guess is that the pins will get warm after an hour at mid-power and start getting a bit hot after 15-30 minutes at full power.
If that is the case, then it's OK to use at lower power settings, but probably best not to run it at full power for more than five or ten minutes.
What can you do about it? If you're happy taking things apart, you could buy a high-current flex (not cable) and replace the flex and the plug. You'll need flex suitable for 13A or more.
If not, and you are still concerned, then maybe get another heater.