With the various anecdotes I've read on CB I wouldn't want to give some yewt or otherwise reason to damage my car. I don't currently live in Utah but I have to imagine that "Repping the Y" on your car in Utah isn't treated by many in typical sports fandom.
The University of Utah or Utah State are likely not associated with the LDS Church in most people's mind even though they are in Utah.
As I have read plenty of people mention before I have to imagine that a lot of Utah fans that are simply Utah fans to hate on the Church and since BYU is part of that representation things move general good natured sports teasing to hating on some ones religion.
Also based upon recent polling it appears that LDS good natured feelings toward others unfortunately doesn't appear to be reciprocal.