Its not safe if a rabbit ingests too much cardboard and paper. It 8s cellulose, but its not proper food. A rabbit should eat lots of hay, if she fills up on cardboard and paper, she will probably not eat enogh hay. Hay is important for both the teeth and the GI function – as I’m sure you know.
Twigs from pesticide free apple or willow or other bunny safe trees (birch, linden aspen, rowan, hazel etc) can be a good cardboard substitute, because they have nutrients and fiber and many rabbits find them very tasty. (They wont replace hay though, theres still nothing that beats hay for a rabbit).
Sometimes excess cardboard eating/chewing is a sign of a dental and/or tummy problem, especially if the bun refuses hay or eats very little of it. Rabbits will often eat more hay if you place several small piles of hay around the bun’s area. They often like eating hay from the floor (or from a fleece blanket etc that is placed on the floor).
You could perhaps restrict her access to cardboard and only let her have cardboard for a limited time every day, preferably a time when you can monitor her?
How do your bun’s poops look?