@daamsie already mentioned the inflatable camping mats, another option is a summer sleeping bag. They can be compressed to a very small size (especially the ones with down feather), their weight is below 1 kg.
Complain to the hotel as soon as possible – it may be a fixable problem.
Some hotels have especially firm mattresses available on request. I once got one of them by accident. I complained, and the next night my bed was much more comfortable.
I don’t think there’s any easy answers here. I’d complain to the hotel if the bed was really that hard. And if you are finding this a regular problem, then maybe bringing along an extra soft layer might be an idea. The small inflatable camping mats pack down pretty small.
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