This is a great idea for those of us who have a "broken" birdbath (no longer holds water). I would be careful, though, to place it somewhere where it would be protected from extreme weather because one big storm and it would be ruined.
Put newspaper over the dirt pages thick & then cover it with mulch. The newspaper will prevent any grass & weed seeds from germinating, but unlike fabric, it will decompose after about 18 months. By that time, any grass & weed seeds that had been pres