The mouse combo performs a song that both is / is about a celebration of their freedom of speech. The mice are interested in testing the boundaries and openly criticizing Garfield because they "can" do it, and do not consider whether they ought to do so, and if it will have repercussions beyond being murdered. As is all too frequent an ailment in fully developed nations with protection of natural human rights, these creatures have confused, ignored, or forgotten that guaranteed freedom of expression does not absolve one from responsibility for that expression. It does rather the opposite.
Sung, presumably, to the tune of "Blue Tail Fly" ("Jimmy Crack Corn"), the mice's song is tied to the history of minstrelsy and the larger tradition of American folk music. It is a protest song of sorts, in the mode of complaint, lament, or criticism aimed at Boss, the Man, the System; the power of these sorts of cheeky-serious numbers has historically been that they are symbolic, coded, or written in slang, and that the ruling class does not see musical expression as a meaningful threat, or does not patter Romany as it were. With this song the mice are probably a little too de-coded and foolishly perform one inch from the oppressor's face.
And so do the singing mice commit an error common among adversaries of the Batman. Criminals aware of Batman's "no guns" and "no killing" rules constantly try to exploit the perceived loophole, and particularly foolish villains will use it to taunt the hero. The self-imposed rules, of course, are flexible at best, questionable for certain. Garfield does not normally do violence to the mice because he has no motivation to do so. The mice seem to have confused Garfield's disinterest with benevolence or weakness.
Meanwhile, in the title panel, Garfield leaves his particularly unappealing bite pattern for forensic odontologists, so it is a good thing he did not eat the mice.
This is all theoretical, of course, because in practice, Garfield crushes and maims mice all the time.