It may have been 25 years ago and maybe even now if you include inactives who never attend. Majority of non-member football fans in SL, Davis, Utah, and Tooele counties all claim Utah. With SL County becoming less and less member by % I really don't know how you could make that claim now. 

I come from a family completely comprised of Utah fans. I grew up going to Utah games in the little stadium that was half full.  4 of my siblings are season ticket holders and sit in one of the "good areas" of the stadium. The BS chants back in the day were pretty funny. My mom would cover our ears whenever they started. There weren't groups of members running over to stop them or booing them to get them to stop. If there was a faction booing, most likely they would have been heard loud and clear. The section that sat in the old wooden benches was a pretty raucous group. We've(my siblings and I) had this discussion and while there are good members who cheer for Utah, it's not the majority anymore and in a game day stadium atmosphere, I would put it at 30% max.