Total Viewership
Over the last 4 years, BYU has 38.4M viewers, which is fewer than Utah(49.6M), UCF (58.3M), Cincy (41.7M), even Memphis (45.2M), Army (48.1M) and Navy (49.8M), and only slightly ahead of Boise (30.3M)

Average Viewership
BYU averages 1.07M per national broadcast.
Cincy averages 1.74M
UCF averages 1.67M
Utah averages 1.55M
Memphis averages 1.51M
Houston averages 1.10M
Boise averages 0.89M

Average Viewership with weak opponents removed
Of course, maybe that's an artificially low average because BYU has so many of their games (70.6%) on national TV compared to Cincy, UCF and others (although UCF actually has 74.5% of their games on national TV, and Boise has 69.4%, Memphis, Cincy, Houston are more in the mid-50's percentages). 

So let's just look at viewership for the top 20 games for each program during that time. That way we can cut out games when BYU is playing a crappy opponent. In that case:

BYU averages 1.51M per game
UCF averages 2.63M
Memphis averages 2.50M
Utah averages 2.19M
Cincy averages 2.05M
Houston averages 1.36M
Boise averages 1.27M

Average viewership for top matchups
But maybe let's just look at the really elite games - like the best 3 per season. So 12 total over those 4 years. Doing that:

BYU averages 1.87M
UCF averages 3.58M
Memphis averages 2.96M
Cincy averages 2.88M
Utah averages 2.84M
Houston averages 1.89M
Boise St averages 1.60M

Viewership against non-P5, non-bowls
And because people always respond that Utah only outdraws BYU because of their opponents, that their ratings show people watching Utah's opponents.  Consider that the same rationale would also hurt BYU since we play better opponents than UCF, Cincy, etc

BYU's best non-bowl, non-P5 draw was 1.21M against CCU in 2020
UCF's best non-bowl, non-P5 draw was 4.7M against South Florida in 2017. They have 11 non-bowl, non-P5 games that have drawn 1.2M+
Cincy's best non-bowl, non-P5 draw was 3.12M against UCF in 2018. They have 7 non-bowl, non-P5 games that have drawn 1.2M+
Memphis's best non-bowl, non-P5 draw was 3.39M against UCF in 2017. They have 6 non-bowl, non-P5 games that have drawn 1.2M+
Houston's best non-bowl, non-P5 draw was 1.39M against Cincy in 2020. That is their only non-bowl, non-P5 game that has drawn 1.2M+
Boise's best non-bowl, non-P5 draw was 1.38M against Marshall in 2019. That is their only non-bowl, non-P5 game that has drawn 1.2M+

Clearly some of this is attributable to geographic location and BYU struggles by being in the Western US, but the end result is the end result. BYU draws substantially lower TV viewership than Utah, UCF, Cincy, and Memphis, is loosely on par with Houston and slightly ahead of Boise St.  Comparing to current Big12 teams shows more of the same (don't have data for all of them, but built TCU, West Virginia and Oklahoma into the database and all three of those teams are substantially ahead of any of the teams listed here, regardless of metric used)