Women just hate feet in general. It's not all self-critique, because they hate guys' feet as well. I feel like most girls automatically envision the nastiest, most-fungal toes they've ever seen when they think about feet, because the culture teaches them that the body part is dirty and gross. It sucks, but most girls just want nothing to do with feet.
That said, I've tickled most of my female friends on their feet and a large number of them have tickled mine. While I'm sure most girls aren't eager to participate in 10 minute tickle-sessions, a quick tickle here and there is never a problem.