In a lecture on the decline of the Mycenaeans, my professor said that Greek society was stunted or retarded due to the collapse of the bronze age. During that time, literacy declined, Linear B was lost, outside trade came to a halt, and people left the major cities. Are all these descriptions true? Are they truly a negative events, or are historians just biased towards these kinds of social changes?
No, that would imply that there is some sort of scale from bad to good that a society could progress or regress on. This is not how the world works. A society changes and changes constantly but it is not along a chart from bad to good. Certain things can be better and other things could be worse. Also what you consider bad and what the historical people consider bad could be wildly different. A good Historian should never try to make value judgments about a society like that, so it sounds like an example of a bad lecture. Things changed a lot, and saying "literacy declined" or "trade declined" is accurate if terse, but trying to say this represented a regression of a society as a whole displays a lack of understanding.