To the LW:

Okay, lady, it seems like you missed an important point here: everyone at your friend's wedding is going to be in cartoon costume. Under those circumstances, worrying that people will stare/laugh at you for wearing such a costume isn't merely self-conscious, it's self-absorbed.

Besides, while dressing up like a cartoon character may be ridiculous, it's fun ridiculous - unlike some Bridezilla-centered affairs. As an adult, you will rarely get the opportunity to do something this silly and have everyone be okay with it. Enjoy it while you can, instead of worrying that it will make you look ridiculous.

Finally, if you do what Prudie suggests and back out of being your friend's maid of honor just because you don't like the wedding theme, you're probably going to damage your friendship with her - perhaps irreparably. True, she should have told you that it was going to be a theme wedding when she signed you on as the maid of honor: you should probably discuss that with her. But don't insult her by throwing a fit and telling her she has to find a new maid of honor. It's not worth it.

she should allow herself to be blackmailed into doing something she doesn't like and does not want to be a part of?

This is just another version of the bridezilla thing, except instead of the ugly dress and shower things, she has to rent a cartoon costume and act against her wishes.

In either case the bride gets her way and the bridesmaid has to suffer.

Under those circumstances, worrying that people will stare/laugh at you for wearing such a costume isn't merely self-conscious, it's self-absorbed.

Not really. What if the bride wanted to get married on a nude beach? It shouldn't matter because Hey, everyone else is nekkid too. ;)

It would be nice to inform people of the theme prior to or during the invitation to be in the bridal party so you can find people with the appropriate comfort level.