Myrtle seems to suggest that there are people who "ought to know", who are of higher stature than Nick and those people are way above the grasp of the common man. This statement could well mean that the society judges people based on their wealth and things rather than the character.
Explanation:
F. Scott Fitzgerald's classic novel "The Great Gatsby" is set in a pretentious fictional world of New York where the people are classed based on their wealth. The high class comprising the old wealthy families reside in East Egg while the lower class is comprised by those living in West Egg, them only recently attaining wealth.
This class structure and the snobbish attitude of those in higher esteem seems to be the theme of the story. Like for example, when Myrtle told Nick that her sister Catherine "is said to be very beautiful by people who ought to know", it suggests the snobbish nature of the people who think they are so called 'high-class'. Myrtle, even though belonging to the lower ranks in the society, began acting way above her level just because of her affair with Tom Buchanan, a wealthy man from East Egg.
Also, the need to assert one's class seems to revolve around the lives of the characters. When she said "by people who ought to know", she is acting as if she is of the same class as that of the wealthy people. She pretends as if she has a class which might even be above Nick's. This pretentious attitude, the need to assert her place, her position in society all show that she or rather the society judges people based on their wealth and belonging rather than character.
