A few highlights from his tenure at MSU.
"and for the first time in school history, a fourth straight bowl berth."
"In 2014, Mullen led the Bulldogs to one of the greatest seasons in school history."
"A week later, they beat
#2 Auburn, 38–23, which propelled them to the top spot in both major polls (as well as the inaugural College Football Playoff Top 25 —the highest ranking in Mississippi State's history, and the highest that any FBS team in Mississippi had been ranked at that late date in almost half a century."
"The Bulldogs finished 10–2 – only the third 10-win season in school history. This netted them an appearance in the Orange Bowl, where they lost to Georgia Tech, 49–34, the third major bowl appearance in school history, after the 1937 Orange Bowl and the 1941 Orange Bowl."
Sounds to me like he's given MSU more success than they've ever seen. I'd say that's a good indication of turning a program around, when you've taken a program higher than any previous coach has before. You don't have to appear in a National Championship to turn a program around.