Pachulke may be on to something.
If someone asked me to define the word 'jock,' I would probably say a man or a boy whose life revolves around sports, who is known for being a successful winning athlete and is generally uninterested and/or untalented in other subjects like academics, politics, music, and so on.
But if someone used the term of a woman or a girl, by extension, as Franzen does, it wouldn't surprise me, and it wouldn't make me think she wanted to be male, just that her narrowness of interest, her singleminded focus on sports, is more typical of men and boys.
That is, the etymology, the image of a jockstrap, isn't present in the mind of everyone who uses the word, just the image of a particular personality type that's well known in high school and college.
The derogatory NOAD sense 'a slow-witted person' can be present to varying degrees. That sense probably comes mostly from American football, where being big and strong is the main qualification for many positions and brains aren't an asset. (Convenient, since those guys are essentially signing up to get concussed, but now more and more people seem to be realizing it can be a form of exploitation.)
I personally would draw the line before 'sports fan'; to me a sports fan is only a wannabe jock, or perhaps an ex-jock, but to be a real jock you have to still be doing (competitive team) sports actively.
I don't know how to express all that within the constraints of the LEO format. To me the primary/original meaning is still masculine, but maybe you could add something like 'weniger häufig auch:' or 'gelegentlich auch:' for the feminine sense.
See what others (AE speakers?) say.