It can depend on the property. Most of the time a sofa bed is included, especially if children are using the sofa bed. But from time to time I have had a property want to charge me to put sheets on the sofa bed.

For example once at the Sheraton Grand Hiroshima I could use the sofa bed with whatever extra sheets/pillows were in the room, but if I wanted it made up by housekeeping it would be an extra charge. Have also seen room descriptions saying an adult using the "extra" bed like a sofa bed/trundle is an extra charge (as it was it had no sheets on it - luckily we had an extra sheet and blanket). As suggested read the room description and contact the hotel for clarification if unclear.