In this context, ‘This’ is the adjective, as it describes the amount of Karens there are in this sentence. Karen is indeed the noun in this situation.

An example of using ‘Karen’ as an adjective would be: ‘did you how so-and-so went completely Karen on that clerk?’

You visualize how so-and-so acted in a particular way towards the clerk. The adjective to describe the behavior was ‘Karen’.

Hope that clears things up.