Sheet Saver Safe?

So I’m a FTM and basically everything I’ve heard is nothing in the crib is best. So all I have in mine at home is the crib sheet itself. However my MiL decided to buy a crib(idk why we live 5 min from each other and I have a pack and play🤦🏼‍♀️) but she bought sheets and these sheet saver things? I’ve never seen them so I’m attaching pictures. But are these safe in the crib? Honestly I don’t think my baby will ever even sleep in the crib there - because why would he? My husband and I both work at home so we don’t really even need a sitter- but just in case I want my baby to be safe!!