I believe I know the study you are referring to and you are sort of right. on a scale of accuracy, slightly depressed people are pretty accurate in self assessments, happy people tend to over exaggerate their abilities, and moderately-severely depressed individuals rate their abilities lower than their actual level. So if the person's average ability was a 5/10, happy people tended to rate themselves a 6/10 and moderately depressed people a 4/10 on average iirc.

Edit: My favorite example of Dunning-Kruger happens to be found in religion. The people that know God the most tend to talk about how little they know God, where as the proselytizers that claim the most to know God, really have no clue about the teachings of christ, mohammed, etc.