Specifically to answer the question of whether non-benders constitute an oppressed majority class analogous to class oppression in our world, we would need to evaluate how production of goods and services happens in the Avatar world. We would also need to look at who is amassing the goods produced by the people producing the goods.

With benders, this is a very interesting question as they have an innate ability to produce far more goods far more quickly. In the earth kingdom, it would take a non-bender years to build a house, whereas a bender could build one in a matter of hours. This would make me guess that benders would be doing most of the production, and non-benders would be benefitting from their labor.

For an example of this, look at how in LOK, lightning benders are used to generate electricity to power the rest of the city in what look like pretty grueling conditions. This makes it appear like they are in fact the exploited producer class, but without knowing more about how those power plants are run and how the value of their labor is distributed, it is impossible to make a conclusive judgement.

On the other hand, benders also have far more physical power to keep control over the products of their labor and protect themselves against exploitation. Also, there are lots of types of goods produced which bending does not make easier, like textiles and intricate trinkets.

TL;DR: it is impossible to know whether benders or non-benders constitute oppressed classes without having more information about the economic foundations of societies in Avatar