Forgive my lack of artistic ability this was a 10 minute sketch colored in MS paint http://fc02.deviantart.com/fs22/f/20...tchka_west.jpg
this is the style I would suggest... the colors aren't very Lolita but if the Loli-gods come down upon you remind them it is a cosplay and not everyone uses it as a lifestyle fashion.
I tried to preserve the elements of the joker suit in the figure you showed I know that the jacket would be a pain in the butt to make but it is a pretty common Loli style.
For accessories a pair of striped stockings would go well with the character ... not Loli but the character itself. I would also suggest a small purple top hat with a joker card and a small toy gun on it that says bang (a small plastic toy you just glue a toothpick and a small sign to)
I look forward to seeing what you do with this