Really? In my experience, they were the best "cheap" airline, by far. Their airplanes were pretty comfortable, compared to United, Alaska, or Spirit, to name a few.
To be fair, comparing most european airlines seems good when you compare them to US airlines. Would be more fair to compare them to other european airlines.
The most famous "cheap" European airline, RyanAir, is substantially worse than Norwegian. And personally I find Norwegian better than WizzAir, easyJet or Vueling.