My 3 month old has a virus and is congested and has had trouble breathing at night when asleep. The nurse at the hospital suggested we raise her mattress to help drain the mucous. Is it safe to do this? Could she suffocate from her mucous as she can only nose breathe.

Red Nose recommends that all babies be placed for sleep on their back on a firm, flat (not elevated) mattress.
There is no evidence to suggest that elevating a baby’s mattress helps to “drain mucus”

Red Nose has the following information about elevating a mattress - most of our information is related to reflux, but the reasons for not elevating remain the same

.........elevating the cot can result in the baby sliding underneath the bedding and is not recommended.
Pillows or positional devices that position the baby with an elevation and are often marketed for baby reflux, are not recommended due to the risks of suffocation and lack of evidence supporting efficacy
https://rednose.org.au/article/why-back-to-sleep-is-the-safest-position-for-your-baby

You may also find our information about bedding for a baby with a cold helpful.
https://rednose.org.au/article/bedding-for-babies-who-have-a-cold

If you are still concerned about your baby, please discuss this further with your GP or Child Health Nurse.