"比较轻柔的摇滚乐" can be translated into English as "relatively mellow rock music" or "softer rock music".
"Relatively mellow" conveys the idea of being somewhat gentle or not overly intense in comparison to typical rock music.
"Softer" directly describes the lighter or more subdued nature of the rock music.
Both expressions effectively capture the essence of "比较轻柔的摇滚乐" in English.