Anyone believing that is overstating things. But anyone stating that Utah's generally good regular season viewership numbers are only due to their opponents is also not looking at reality.
It's obviously hard to compare apples to apples on viewership draw with a school like Missouri (who gets SEC opponents) or BYU (who played an Indy schedule) or Memphis (playing a G5). But it's pretty easy to compare Utah to Arizona, Arizona St and Colorado and Utah comes out favorably in those comparisons.
There's legitimate arguments against Utah to the Big 12... But lack of fanbase/viewership isn't a well-supported one. And to keep claiming it as a weakness of theirs (but not of the rest of the four corners) only serves to paint us as biased and serves to diminish our more accurate critiques