Don't feel bad. You did what you felt (and probably WAS) best for your family. It's a good idea to have all requests come through the caseworker so she can sift through them and determine which ones are reasonable and which aren't. It's obvious the birthmom isn't able to do the filtering herself, and as you've discovered, you don't need that stress right now!
If you feel like it and have time, you can get professional pics taken (at Sears or wherever) in the outfit she likes, and give them to her as a Christmas surprise. You can have the caseworker give them to her so the caseworker can stress that requests are still to be filtered through her first, but that you thought she would enjoy these pictures.
But, you are under no obligation to do so at all. It's just a nice thought if you don't think it would open up a can of worms
.