"比较轻柔的摇滚乐" can be translated into English as "mellow or softer rock music" or "gentle rock music".
"Mellow" implies a smooth, relaxed, and not overly harsh sound.
"Softer" directly contrasts with the heavier, more intense aspects often associated with rock.
"Gentle" also conveys a soft, unobtrusive quality.
All these expressions effectively capture the idea of rock music that is relatively light in tone or intensity.