They don't detract at all because they're true. McCain has some serious anger/temper problems and that's not really a secret. He's had numerous outbursts on TV and on Senate floor.
The measurement doesn't need to be objective. If you agree with the journalist, then this statement supports his general argument. If you disagree, it doesn't. That's all.
Now, there are ways to measure anger objectively:
Cortisol in bloodstream;
volume of voice;
Blood pressure;
etc.
In fact, the amount of cortisol in the bloodstream is a rather all-encompassing measurement that works as a pretty good proxy for how angry you are, even though it is technically your level of stress.
http://www.boston.com/news/nation/articles/2008/01/27/famed_...
So I don't really why stating that simple fact would take away anything from its credibility.