I've had a bit of a busy day, so I apologize for not getting on this blog sooner. Thankfully, I can now.
An official Gravity Falls book says that Mabel would want the power of invisibility because it means she could go naked and no one would mind (mainly because they couldn't see her). This picture is proof of that. It's also really good because it ties into the goofiness of little kids, and the reactions of the cast are all in-character: Dipper's disappointed, Stan's shocked, and Wendy's amused. I've always wanted to feature Mabel on this show, so I'm glad I get to do so now. A pity the art I really wanted to post can't be. Ah well...