Obtuseness is the quality of being obtuse: dullness of mind — slowness to see the point, or a blunt refusal to see it. That is the sense you will meet most often, and it is not a kind word. Though actually, writers sometimes stretch it toward emotional bluntness: not “they cannot do the sum,” but “they cannot feel how their words land.” The word is formal. In everyday talk, a friend would say “he's being thick” or “she's not getting it,” not “his obtuseness.” Maths can use obtuseness for how far an angle sits past 90°, a dry use with no insult in it, but that is not the usual people-sense of the noun.