Good point! I would LOVE an app like that.
Several years ago there was a denim site -maybe "girl shop"? -that used real (rather than computer-generated) women who had a variety of body-types. You could select a model whose body was close to your basic shape and then see what various designer denim styles looked like on her.
I also remember the old Land's End virtual model - it wasn't great, but it did help give a sense of what something might look like on my tall pear shape.
Bottom-line: Surely the underlying technology has improved in the past several years-somebody needs to do this! (I'm looking at you, Nordstrom.)