12th grade AP stats grades slipped, what's the best way to write to my college I've been accepted to about this slip?

I would get in touch with their admissions office, preferably by phone call (since those are usually more humanizing than email), and make the conversation about why you were struggling so badly in your math class—maybe you and the teacher weren't compatible, or the workload was too much with your other classes, or something like that. But make sure you have reasons you can give them beyond just having a bad grade.

And also—this is important—make sure you know what you'll be replacing it with. Schools will usually be more receptive to hearing that an admit is dropping a class if they're replacing it with an interesting elective (or even a lower leveled version of the same course) than if they're dropping it to get a free period or more study hall.

I would also enter the conversation by saying "I'm considering dropping this course because I'm struggling/it's negatively impacting my work in other classes/etc." That way you're asking them "is this okay?" versus "I'm doing this, please be okay with it," and they'll probably be more receptive to the former.

Also—yes, they will still care about math even though you're a liberal arts student. They'll require you to take some credits in math before graduation. That doesn't mean that this will be a huge deal to them, but you do definitely need to tell them if you're dropping the course.