。
She wore a sullen expression.
Literal
[topic-は] sullen-とした expression [object-を] was-displaying.
むっつり is a 擬態語 (state mimetic) for being sullenly silent — a brooding, withdrawn quality without speaking. むっつりとした is the attributive form (state mimetic + と + した), modifying 表情 ('expression') — 'a sullen-looking expression.' The collocation 表情を浮かべる ('display/show an expression') uses 浮かべる ('float to the surface') metaphorically — emotions surface on the face. Often paired with 微笑み (smile), 怒り (anger), 涙 (tears), and other facial-emotional content. The whole sentence has a literary observational quality, the kind of line that opens a paragraph in narrative fiction.