I never said he was mediocre. 300+ wins and 5,714 strikeouts would disprove that fairly quickly.
In the eighties, though, he really only had two excellent seasons -- 1981 (11-5, 1.69, a no-hitter, 140 K in a strike-shortened year) and 1987 (2.96 ERA, 1.14 WHIP and 270 K, despite a very hard-luck 8-16 record). He never won more than 16 games in a year. His ERA was consistently around the league average, besides the two seasons I mentioned. And he had no top-3 Cy Young award finishes. Yet he's one of the three best pitchers that decade? Two of the guys I would throw into the discussion, Bret Saberhagen and Orel Hershiser, aren't even on the list!
As for having a lot of numbers...well, when you pitch for 27 seasons, you're going to rack up a lot of numbers. He should have had a lot more than 324 wins for being such a great pitcher. Plus you have to look at some other numbers he racked up, like the all-time walk record (by over 960!!), or the fact that he has 3 of the 9 total 180+ walk seasons since 1900.