。
She gets in a huff at the drop of a hat.
Literal
She [topic-は] easily in-a-huff becomes.
ぷん (or ぷんと) is a 擬態語 capturing the sharp little exhale and stiff body language of someone suddenly taking offense — being miffed, getting in a huff, taking umbrage. ぷんとなる is the change-of-state version: 'to get into a ぷん state'. Related: ぷんぷん (sustained anger, 'reeking of anger'), and the homophone ぷんと in odor contexts (the sharp note of a smell). すぐ ('easily / readily') here is habitual rather than temporal — the sentence describes a temperament, not a single moment.