Absolutely! They have beautiful bark as an interest, do very well at having a nice root flare, although root training will still be necessary. They have beautiful bark as an interest.They back-bud nicely, and they grow like crazy, so they are easy to ramify. (When I am focusing on development, I purposely keep mine from blooming. I keep cutting back the growth that would bloom, and direct the energy into the branches where I want that growth.)
Adam's art and bonsai blog has a great article called Crepe myrtle bonsai: Can you "grow" one?
One of the things that so delighted me when I got into bonsai was that I could grow crepe myrtles, one of my favorite trees. I had moved to the Pacific Northwest from California - and found that they do not particularly like snow. With bonsai, I have wintering space set up for my temperate and tropical trees, and bonsai are easily movable!