This is the trophy for making a custom character with a cape.
Go to Edna Mode's house on the SW edge of Upper New Urban and enter through the door to the right. From there step up on the button and press circle to drop into the maker.
First you must pick which of the 10 spots you want to create your character. Pick an empty one (or one to replace if you already used them all) and the customizer options will appear.
Go to capes and select one of the 3 styles of cape. Edna will begin talking about her poor opinion of capes almost immediately and the trophy will pop. You do not need to save the creation.