Surely the “joke” is that the wearer of the hat is a bit of a gold digger (sorry if I’ve misunderstood the meaning of “footballer’s wife”).
Both males and females do marry other people for their wealth and fame.
The hat does not imply that all females are gold diggers (or even that gold digging is a bad thing).
The hat isn’t sexist, the joke is just quite poor. (especially if you have just bought it for your 6 year old daughter).