Boeing is unquestionably a major one, but you cant not blame customers when their pilots dont know how to fly an airplane outside a very narrow window of happy path.
If you want to talk about proportions of blame:
Lets see how broad this happy path normally is and in case of the MAX:
-> Global aviation crash rate 2012-2017: 0.00000024 per flight (-> one crash per 4.16 million flights)
-> Of these, likely contributed by pilot failures: 0.00000021 per flight (-> one crash every 4.76 million flights). This contribution reflects the average pilots proficiency.
-> This leaves the following difference to be contributed by technical failures: 0.00000003 per flight (-> one crash per 33.3 million flights)
The MAX had this crash rate: 0.000008 per flight (-> one crash every 0.125 million flights -> 33 times worse than the global aviation crash rate in the 5 years before the MAX)
As the same pilots were flying the MAX, that contributed 0.00000021 crashes per flight normally, we can take the 0.000008 minues the 0.00000021 to get the contribution of the MAX itself (components contribute to the total failure rate by simple addition). So we get 0.00000779 as the crash contributing failure rate of the MAX. Which is 260 times higher than the 0.00000003 crashes per flight, which are normally contributed by technical failure.
So, as you say, the MAX occasionally has left a very narrow window of happy path for the crews to react. 260 time narrower than with other aircraft so to speak. But we also have to say, that the subpar airlines were likely the first ones to expose this weakness of the MAX. And that migh be in line with this report about ET.