。
She deliberately showed off her dress.
Literal
[topic-は] deliberately dress [object-を] showed-off.
見せびらかす is a derogatory-tinted verb for 'show off, flaunt' — formed from 見せる ('to show') + びらかす, a suffix conveying ostentation. Unlike the neutral 見せる, this verb carries an evaluative judgment: the speaker thinks the showing-off is excessive or distasteful. Combined with わざと ('on purpose'), the sentence reads as someone making a calculated display, perhaps to stir envy. ドレス is a loanword (from English 'dress') referring specifically to a Western-style formal dress, distinct from native words like 着物 or 服.