When I purchased my Detours USA rear bumper with tire carrier, I asked them to make it where the spare tire was at an angle that would match the back upper hatch. They agreed, and said they have never had that request before.

So it's really just a preference thing. I would expect it to be at 90 degrees or less to the rest of the XJ of course. If greater then 90 degrees then I'd say someone messed up when they built it.