Never an elite running back
One could argue that at the height of Joe Mixon's time with the Cincinnati Bengals, he was never a Top 10 running back. In 2018 he was 4th in the league in rushing yards, but Chris Carson was 5th. Carson was never an elite running back, so we can point to Mixon's first "good" year and say with certainty that the amount of yards doesn't equate to talent. Mixon was 9th in 2019, 3rd in 2021, 27th in 2022 and 8th in 2023.
Now in those same years, Mixon was 8th in attempts for 2018, 6th in 2019, 3rd in 2021, 17th in 2022, and 5th in 2023. That gives you about a 4.3-yard-per-carry average. Not bad. Not elite. Christian McCaffery averaged 5.4 yards per carry in 2023. Nick Chubb, for his career, never averaged less than 5.0 yards per carry.
Mixon is no slub, but he's not the elite runner the team had a chance at in free agency. He's a solid running back, but solid isn't enough when you're talking about the kind of money the Texans are paying him.