Does EasyJet do well in NPS? According to npsbenchmarks.com EasyJet ranks at a -16. Not good. This is far below the mainline carriers such as British Airways, Lufthansa, or other carriers In Europe. But when your point of comparison is US domestic providers they kick it out of the football arena (or at least through that net thing at the end of the field). Did they have a good day? Maybe. But based on my experience traveling both in the US and Europe, air travel in Europe is a dream compared to the United States.
According to the same site, all US providers are in the positive side of NPS with Southwest at 62, Jet Blue at 59, Delta at 41, United at 10, and American Airlines at 3. Would it be fair to conclude EasyJet has worse service than all the mainstream US providers? Based on NPS alone you might be tempted to say yes.
I would argue it is an unfair and unwise comparison. In fact, this is one of three fundamental reasons why cross-cultural comparisons of many attitudinal metrics (including NPS) are fraught with problems that make their comparison problematic.
http://customerthink.com/a-cx-no-no-cross-culture-nps-comparisons/