Now you might say that ERA or ERA+ can be affected by fielders, so if a guy has bad fielders, it hurts his ERA.
But Morris had good fielders like Trammel, Whitaker and Chet Lemon behind him. For most of his time with the Tigers, they were above average in Defensive Efficiency Rating and several times were number 1 (I think 4-5 times). So it seems unlikely that his ERA could have been hurt by bad fielders.
So then when we adjust his ERA for park effects with ERA+, he managed only one top 5 finish. So he was rarely one of the elite pitchers in the league.
Not having any top finishes in FIP ERA might not be his fault if Tiger stadium was an easy park to hit HRs in. But he actually had only one top 10 finish in his career and that was a 6th. Don't forget that walks and strikeouts affect FIP ERA along with HRs. But anyway, we can always go back to ERA+, which, again, does not seem to be unfair to him since it seems like he had good fielders behind him.
He also had only one top 5 finish in strikeout to walk ratio
Now some people said he was a winner. But his career winning pct of .577 is only 205th best all-time. He also had only three top 5 finishes and never led the league, even though he pitched for some good teams.
Some have said he could pitch to the situation. But here are his OPS allowed stats for Low, Medium, and High Leverage situations
Now maybe the norm for starters is to do much worse in high leverage cases. But this does not look that impressive
He was supposed to be a big game pitcher. But in 92 post-season innings he has an ERA of 3.80. Good, but not great.
The table below shows the Tigers Defensive Efficiency Rating each year from 1979-1990, the years when Morris was a regular starting pitcher with them. It also shows where they ranked in the league each year as well as the league average. Then averages for all three are shown. He had some good fielders behind him.