We discussed this a couple offseasons ago. I guess none of the helmets have gone through an extended period of mediocrity, but it's almost like once college football became a national sport, instead of a regional one, in the 70s and 80s, it somehow locked in the helmet schools.
Florida State and Miami are an interesting case study, because they were there only at the end of that, so is their status more fluid?
What defines mediocrity? Lack of conference championships? Final ranking? BCS/NY6 bowls?
Michigan last won a Big Ten title in 2004. Since then they've had two AP Top 10 finishes (2006 and 2016), and 4 BCS/NY6 bowls.
Looking at some other helmet or potential helmet teams, and their worst 15 year stretch in the past 50 years (going back to Bo's hiring). Obviously UM could still accomplish a bunch of things this season
Michigan - 2005-2019 (0 Big 10 titles, 0 division titles, 2 AP Top 10 finishes, 4 major bowls, 3 losing seasons)
Alabama - 1993-2007 (1 SEC title, 3 division titles, 3 AP Top 10 finishes, 2 major bowls, 3 losing seasons)
Oklahoma - 1987-2001 (2 Big 8/12 titles, 1 division title, 1 National Title, 3 AP Top 10 finishes, 3 major bowls, 3 losing seasons)
Texas - 1978-1992 (2 SWC titles, 3 AP Top 10 finishes, 3 major bowls, 4 losing seasons)
USC - 1987-2001 (5 Pac 10 titles, 2 AP Top 10 finishes, 5 major bowls, 2 losing seasons)
Ohio State - 1987-2001 (3 Big 10 titles, 3 AP Top 10 finishes, 3 major bowls, 1 losing season)
Notre Dame - 1996-2011 (1 AP Top 10 finish, 3 major bowls, 4 losing seasons)
Nebraska - 2004-2018 (0 Big 12/10 titles, 4 division titles, 0 AP Top 10 finishes, 1 major bowl, 5 losing seasons)
Tennessee - 2004-2018 (0 SEC titles, 2 division titles, 0 AP Top 10 finishes, 1 major bowl, 8 losing seasons)
Penn State - 2000-2014 (2 Big 10 titles, 1 division title, 3 AP Top 10 finishes, 2 major bowls, 4 losing seasons)