蛮 as 'quite/pretty' is colloquial and very common in southern Chinese dialects and everyday speech. It's softer and more casual than 很 (hěn).
usage
蛮 has two distinct uses: as a colloquial adverb meaning 'quite/rather', and in literary/formal contexts meaning 'barbarous' or 'unreasonable' (蛮横, 蛮力).
