较 is a softening comparative adverb meaning 'relatively / comparatively', so 较差 is gentler than a flat 很差 (very bad). It's a polite way to say something is below average, common in reviews, reports, and feedback.
mistakes
差 has two readings: chà (mediocre / lacking) here, and chā (difference) in 较差 meaning 'range / spread' (e.g. temperature range). Same characters, different meaning by tone.
