。
She took my joke seriously.
Literal
She [topic-は] my joke [object-を] seriously made.
本気にする is a fixed expression meaning 'take seriously' — literally 'make into 本気 (real spirit / seriousness).' The frame Xを本気にする means 'treat X as if it were the genuine article.' Often used when something said as a joke, hyperbole, or rhetorical flourish is taken as a sincere statement — exactly the awkward situation the sentence describes. The opposite is 冗談にする ('treat as a joke').