You know what else is only designed for tall skinny girls? Almost every piece of women's clothing on the planet! So you have two possible choices here. One, you can choose to just confine yourself to the incredibly limited section of clothing in the world made for people with your exact body shape (and I know it's limited because for some reason people forget how many body types there are and just design for the same one over and over again), or two, you can forget who the designers intended it for, take it for yourself, and rock it!
Don't worry about whether or not something is only for "pretty and fit girls." (And I quote that because I'm sure you're very pretty, and I haven't the slightest clue why we use thinness as a barometer for that, and because for as much as we associate "thin" with "fit" I bet you're in much better shape than my narrow behind anyway). To answer your topic question: Of course you can wear this costume. You can wear anything you want. If someone doesn't like it, that's their problem, not yours.