Deliberately obtuse, maybe? Deadpan is probably better, but I just want to put it out there.

deliberately, adv.: with full awareness of what one is doing : in a way that is intended or planned

obtuse, adj.: stupid and slow to understand, or unwilling to try to understand:

If you put the two together, you get something along the lines of: Intentionally unwilling or slow to understand [for the purpose of being humorous or uncooperative]

(The example in the definition for obtuse (link 2) actually uses the construction 'deliberately obtuse.')

But I would still use deadpan because it's a) more common, and b) the humorous intent is more overt and not implied.

Deliberately obtuse, maybe? Deadpan is probably better, but I just want to put it out there.

deliberately, adv.: with full awareness of what one is doing : in a way that is intended or planned

obtuse, adj.: stupid and slow to understand, or unwilling to try to understand:

If you put the two together, you get something along the lines of: Intentionally unwilling or slow to understand [for the purpose of being humorous or uncooperative]

(The example in the definition for obtuse (link 2) actually uses the construction 'deliberately obtuse.')

But I would still use deadpan because it's a) more common, and b) the humorous intent is inherent rather than implied.