How unusual is the 737 MAX 8's crash history?
Historically, it's not unusual at all to have many accidents of an aircraft type in relatively short order, especially when the type is still new and hasn't had a lot of operating hours across the fleet to detect and work out teething problems.

BUT, and that's a big BUT, in recent history that's been happening less and less as testing and certification programs have gotten ever longer and stricter (in large part as a result of those earlier crash series).

As for the 737Max, according to Wikipedia some 350 have so far been delivered (out of over 5000 on the order book) over the space of only 2 years (yes, they're that new). Of those 2 have gone down. That's not a particularly high number historically, but as said in recent history people have come to expect nothing bad to ever happen because there have been so few accidents overall and most of those easily explained by external factors (MH17 being shot down, for example).

So this number is both highly unusual, in that it's not something that's been seen in recent memory, and rather normal, in that in the history of aviation it's not unusual at all.