No, it's a Mach microkernel (except it's not because they shoved some non-microkernel stuff in) with a BSD userland, and a NextSTEP GUI/framework on top.
As I understand it, the kernel is pretty much a hybrid of Mach and FreeBSD, and I don't think you can really say that it's more of one than the other: http://en.wikipedia.org/wiki/XNU
There's definitely more BSD in there than just the userland.