When I listened more carefully, it turned out it wasn't a joke — they were genuinely clueless.
Literal
Well listened-[conditional-たら], joke [not-じゃなくて] genuine-cluelessness was [apparently-みたいです].
ネタ ('material, joke, bit') is entertainment slang — here, what they thought was a comedic bit. マジボケ ('genuine cluelessness') combines マジ ('serious, for real') with ボケ ('the funny person in a comedy duo, or a clueless mistake'). みたいです ('it seems, apparently') softens the revelation. The sentence navigates the comedy/reality boundary that's central to Japanese humor culture.