I don't like the outfit at all. It's extremely ill fitting and doesn't make sense, not its cut, length, size. The big zero print is awful, too.
I don't think it's so special that she supports Britney either. Britney doesn't need her support because commercially she's been many times more successful than Katy already. It's really one of those occasions on which you get mail which reads 'the pleasure of your company is requested'; Katy's fulfilling her societal duties, that's all.