The mermaid tail is actually intended to go behind her, not in front like an apron, though of course it can be used any way you like!
From the website:
The WellieWishers love to play dress-up—and Camille’s favorite set is a mermaid-maker. The shimmery fabric fin fastens around a WellieWishers doll’s waist and trails behind her, so she can pretend she’s a mermaid! The set also includes a chest to hold a mermaid’s special treasures: a tiny message in a bottle, an hourglass, a pretend seashell, and a seahorse pillow.