Adding mulch inside raised beds is unrecommended and highly discouraged. You might have seen some people recommending it, but believe me you should never add mulch on your greenhouse raised beds.
Usually, mulch is added onto a greenhouse raised beds to increase the humidity and to prevent weeds from growing inside your raised beds.
But inside your greenhouse. it is already very humid most of the time.
So, if you add mulch inside your greenhouse you will be providing a sanctuary for pest and disease to thrive. Therefore, you should never use much in your greenhouse raised beds.
See, our greenhouse veggies growing guides for more details. We recommend using ground cover fabrics for this purpose.