...I guess that depends on where someone would rate him. If you view him as the greatest strikeout artist ever and one of the most intimidating pitchers of all-time, then no, you're not overrating him. He led the majors in K's 11 times, struck out 300 batters 6 times, struck out 200 batters 12 times, and played an astounding 27 seasons. He holds the career records for K's and Hits/9 and is in the top 15 ever for total wins.
He was a great pitcher and a clear Hall-of-Famer. Saying that much isn't overrating him. Now, putting him up there with Randy Johnson, Tom Seaver, Greg Maddux, Bob Gibson, Walter Johnson, Cy Young? Yes, THEN you are overrating him.