Yes, it was an invitation, but the mistress of "distress" has already accepted. In all fairness, the bride and groom should have let people in on this idea before asking anyone to join the wedding party. That would make it easier to decline.
I am thinking they must not be all that close of friends if the bride didn't have an inkling of how she might feel. I didn't think Prudie's answer was too bad, but the bowing out needs to be done quickly so the bride has time to find someone else.
I would probably go along with the cartoon thing myself after asking "What!? And are you sure everybody else will look this stupid, too?" Prudie is right, though. You might just come out cheaper with the cartoon thing. I think I could put together something like a Minnie Mouse costume fairly cheaply. At least I could wear it for Halloween later instead of being stuck with some awful stuffy dress that is probably over-priced.
It does seem a bit silly and doesn't make it seem like a wedding as much, but if that's what they want... I hope the couple doesn't regret it later.
I was thinking the super-hero or Star-Wars theme sounds better than cartoons. But yeah, I would do it. I don't care much for big, formal weddings anyway.