Person being crushed is protestors/dissenters that is being crushed by a large oppresive whale (police/media/counter protestors) for speakingout against the whale. Other person comes allong to help, raises their voice, and is told to "keep things civil" as if they are the instigators, while the whale is maliciously using its weight to stiffle the oppressed.
Basically the norms and establishment of society is large and unwieldy and can crush any oppsition with no way of retaliation. And if somone points it out, dispite being equally small and cannot truly harm the whale, they are the ones out of line and instigators.
Cat is being crushed by the whale. Girl repeatedly asks the whale to stop crushing Cat, getting louder each time. The whale ignores her until she shouts at it. The whale finally acknowledges Girl, but only to reprimand her for not keeping the "discussion" civil.
The joke is that whale ignored her pleas until she shouted, but instead of addressing the content of her pleading, all the whale cared about was the perceived incivility of being loud.
An additional layer to the joke is that the reason Cat is being crushed by the whale is because Cat complained about being crushed by a whale.
As an allegory, it's applicable to scenarios where one group abuses another group, and then punishes those victims for complaining about the abuse by victimizing them further -- and when called out on it by a third party, the abuser does not address the abuse, but instead complains about the tone of the call out.
What specific, real-world situation this strip refers to is open to interpretation, since it's illustrating a typical dynamic of severe power imbalances.
(But my own errant speculation is that it was inspired by the crackdown on pro-Palestinian demonstrations at US college campuses. YMMV.)
