。
She was wearing heart-shaped earrings.
Literal
She [topic-は] heart-shape [genitive-の] earrings [object-を] was-wearing.
イヤリングをする ('do earrings') is the standard collocation for 'to wear earrings,' with the auxiliary する handling the act of putting on / sporting accessories. The same する covers ネクタイをする (wear a tie), 指輪をする (wear a ring), and even 化粧をする (wear make-up) — anything you 'do' to your appearance with an accessory or cosmetic. ハート型 attaches the suffix 型 ('shape, type') to a katakana loanword to form the compound 'heart-shaped'; this ~型 is highly productive for describing forms (V字型, 'V-shaped'; 大型, 'large-type'). The past progressive ~ていた sets the scene as observed at some past moment.