CR-21 is a self-buffing AR, with her Skill increasing her Damage.
ARs have been powercrept repeatedly over the years, the use cases for ARs have shrunk, and almost every AR that does not have a strong Mod is not worth using. As such, CR-21 is not recommended for use.
Full analysis
CR-21’s main selling point is the unique targeting she possesses on her skill, Muscle Pump. For the duration of the skill being active, she will focus her fire on the enemy with the highest damage. This means, like RFB, she has the ability to snipe Jaegers before they become a threat in echelons they are present. In general, this targeting will mean she will focus on the most threatening enemy first regardless.