The ratings are not used by the game. They are intended as a quick guide and in general guys with better grade/clutch ratings are better pitchers.

When you look at specific pitchers though, ignore the grade and look at his actual stats plus LOPS/ROPS. I think things like walks per IP, HR per IP are also useful info.

I don't think anyone understands the clutch rating [if someone does, I'd be interested in their ideas], but my WAG is that a pitcher whose ERA is much better than their stats would predict gets an adjustment that is represented in the clutch rating. I don't think the game uses the CLUTCH rating, it is again just a quick & dirty number to show how a pitcher does with men on base.