The target audience are the corporate boxes, filled with people my age who 15 years ago could still pretend to themselves they were (we weren't, but could still delude ourselves so) and Katy Perry is about as modern as music knowledge goes for many. Then they have to pick somebody reasonably mainstream or risk alienating too much of that audience.
Plus, she has done the MCG for a quick set before and after a major event before without "Loafing it" (yes, that's a term). Not my thing, but a solid enough choice given the big-ticket target audience is corporate box, middle aged (and older), theatre goers demanding a recognisable name, rather than sub-30 year old football fans who are mostly locked out on the day anyway - and certainly not paying thousands a head for their chance at bribing a client or two.
Plus the Grand Final is a place careers go to die anyway.
