- Making one unable to think or feel properly.
‘the game ends with a sense of stupefying boredom’
1.1 Astonishing and shocking.
‘stupefying financial incompetence’
In particular the 1.1 definition entails the "surprising" quality you're looking for.
Pedantically, stupefying implies that it makes you (the observer) stupid, rather than whoever created the thing.
But there is a lot of leeway here as we're dealing with exaggerative language.
Colloquially, it is often used to mean "with surprising stupidity", in the sense that it was so obviously stupid that I couldn't comprehend how they hadn't yet realized how stupid it is.
I'm trying to express not that the level of stupidity is surprising, but the presence of it being surprising.
This fits with the colloquial usage of stupefying.
Also note that the etymology reveals a similar intention:
from Latin stupefaciō (“strike dumb, stun with amazement, stupefy”), from stupeō (“I am stunned, speechless”)