As I understand current thoughts about babies and quilts is that the crib dressings should be very minimal. Blankets/quilts not recommended for the first year, just sleeper suits. You can certainly make a fitted mattress cover, just make sure it is ample and secure.
In my Tuesday group, most of the ladies belong to a quilt group that has a preemie project that has some very strict guidelines, like they want the tops pieced to some extent, but we should be aware of the lumpiness of seams, like especially at star points. You might want to keep that idea in mind.
When my son was little, I do remember one night vividly. Poor little guy was spewing from all ends and repeatedly. After the second full changing of sheets and bumper pads, I got out the big beach towels and just wrapped fresh ones of those around the mattress. Since then, I often give a large fun beach towel as a shower gift with a telling of the story and just the all around usefulness of such an item in your car or diaper bag.