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.