Hi Lisa!
I've taken my daughter to
Bibbidi Bobbidi Boutique a couple of times, and I would say you're right in that most of the dresses were Cinderella, Belle, and Aurora (Sleeping Beauty). There were less Ariel dresses and even less Princess Jasmine and others. There's also the issue of sizes, where they may have her favorite princess dress, but maybe not the right size for her.
What we did was purchase our daughter's dress before the actual trip and packed it up. We brought it to the Boutique appointment, and they allowed my daughter to change into the outfit we brought before she got her hair and face done.
There is also a store right across from Cinderella's Castle that sell Princess dresses. The price is the same as it is in the Boutique, so there's another option.
Hope your Princess has a magical 5th birthday! Be sure to get her a free "Happy Birthday" button when you enter the park, to make her day extra special!