Katy Perry isn't what I normally listen to (I'm a metalhead!! ) but I thought the sound was spot-on. Atmosphere-wise you're almost never gonna get the entire crowd rocking along, even if it was at twilight. Most music acts have their fan base, and your average footy crowd crosses too many demographical lines to ever rock out as one.
Her voice was okay but she's not a "powerful" singer like Christina Aguilera or Kelly Clarkson, and the MCG is a nightmare from an acoustics perspective anyway so it just sounded thinner than it should have