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Her clothes weren't expensive, but they looked clean and tidy.
Literal
She [topic-は] expensive is-not [but-が] tidy outfit [object-を] was-doing.
さっぱりとした ('refreshing, clean, tidy, uncluttered') is built on the 擬態語 さっぱり, which captures a feeling of clean simplicity — applied to taste (さっぱりした料理 'light, refreshing food'), personality (さっぱりした性格 'frank, no-nonsense'), and as here, dress style. The と+する formula adjectivizes the mimetic. Clause-final が works as a soft 'but,' sliding from the negative observation into the positive one. The whole sentence delivers a small character sketch — modest taste with care.