you shouldn’t put anything under the mattress but you can put a book under one end of the feet of the crib to create an angle, it ensures it’s still a completely flat and level surface whilst creating the slight elevation to help with reflux!
@Chloe Ah that’s a good idea!
Some next to me can be tilted, as in lower/raise one end. Just make sure it's a very small tilt, or she'll slip to the bottom