No need for an extra mattress

The thin, firm folding mattresses supplied with folding cots are designed to provide good comfort for a baby.  Do not put an extra mattress in the cot. 

Too much or too soft bedding can be dangerous to babies. Infants can be smothered when caught up in extra bedding as these items can trap children, fold around their mouths and prevent breathing. These items can also cause overheating.