“It has long been the most affluent neighborhood in New York City.”
“The Upper East Side maintains the highest pricing per square foot in the United States.”
“Four of the top five ZIP Codes in the nation for political contributions are in Manhattan. The top ZIP Code, 10021, is on the Upper East Side and generated the most money for the 2004 presidential campaigns of both George W. Bush and John Kerry.”
Yes. That's basically east of Central Park at the southern end of museum mile. So, generally, a very good (if somewhat sleepy) neighborhood.
It's what most people would consider so, yes. It's an upper-middle-class area, clean and safe, if more than a little stuffy and uptight.
That's an understatement. It's consistently one of the wealthiest, if not the wealthiest (depending on the year) locations in the entire country.
It is, depending on the year, how you measure it, and what you consider "Upper East Side". That last part is important: the UES has three main subneighborhoods with very different demographics and characteristics. It's also adjacent to East Harlem, which is historically not wealthy and brings the mean/median down for any measure that includes part of it.
For example, here the Upper East Side is listed as the wealthiest neighborhood in the entire city, all five boroughs: https://ny.curbed.com/2017/6/27/15881706/nyc-richest-neighbo.... This is using household income, which is one measure. Alternate measures include median rent (skewed due to rent regulations), net worth, property values, etc.
Point is: calling the upper east side an "upper middle class" area is an understatement. If it's not a wealthy neighborhood, then nowhere is.
Also it's a miserable part of the city to live in.