Clevinger was actually pretty good until September. His bad September and October probably cost him a chance at a multi-year deal.
Being that it's his first year back from TJS, it could be that he just fatigued late in the season. He's got a 114 inning base to work from for 2023, so if he stays healthy, I don't think fatigue will settle in until the 150+ inning mark.
The glass half empty take on it is that his decline in September and October are a sign of a further decline. That might be why he only got $8 million. Teams could be skeptical of how his arm will hold up.