Is "below par" good or bad?

I realize a lot of English expressions derive from sports: "his presentation was a slam-dunk," "she really fumbled through that," or "that's pretty much par for the course."

I don't play golf, but I believe "below par" means really good. But doesn't describing something as "below par" mean really not good?

I don't believe the general use of this expression comes from the golf use - which would explain the discrepancy - though the two are closely related.

The word par has many definitions, including average or normal amount and usual/accepted standard.

Therefore something below par is below average, or below the acceptable standard - hence the usual, negative, meaning of the phrase.

In golf a hole's par is the number of strokes considered average or standard to complete it, and being under par means using fewer strokes than average - which happens to be a good thing, because of how the game works.

You have identified a discrepancy, but once again this is a case where the context is all important.
Given that par means average, the goal in golf is to finish using the least number of strokes, so in this case below par is a good result.

If we were talking about soccer, and par was the average number of goals that a team has scored in each of their games, then below par would be a poor result and above par would be a good result.

If we are to generalise and assume that most people would use an ascending scoring system of measurement such as percentages where 100% represents full marks, then below par would be considered a poor result.

However as discussed above golf uses an inverse scoring system where lower scores represent better performance, and therefore in this case below par is a good result.

There are already several good, correct answers here, but I would like to point out one subtle distinction in my observations of idiomatic use of par, other than in golf: I find that par usually has a connotation closer to "sufficient" rather than "expected" or "average".

Thus, when someone describes something (such as a hotel room or meal) as above par, they usually mean "more than what was strictly required, but not necessarily outstanding". In contrast, when someone describes something as below par, it very often means "that was so bad it didn't even meet the minimum requirements". The phrases are asymmetrical in intensity.