Mississippi State team from 4 years ago? Yeah, 2017 sucked, I get it.
Last season they allowed 17 points against Boise State and 22 points against Coastal Carolina.
In 2018 BYU held #6 Wisconsin to 21 points.
In 2019 BYU gave up 27 points to USC in an overtime game and 25 points to #15 Boise State.
The only ranked teams that have scored 30+ points against BYU over the past three seasons have been Utah and Washington.
In 2019, #14 Utah scored 30 points, but only 16 of those were scored on the defense (they had two pick 6s).
In 2018, #17 Utah scored 35 points, but 7 of those came on a pick 6.
So yeah, BYU's defense has underperformed against Washington, but I don't see any other instances where they were dominated by ranked teams over the past 3 years.
The legitimate complaints are the Utah State game in 2018, and the Toledo, USF, and Hawaii games from 2019. None of those teams were ranked.
It's time to change your narrative based on games that happened 4 years ago.