Not a common term nowadays, I don't think, but a "rat race" typically designates some sort of meaningless/endless pursuit, like a race for money, or power, or to get away from a predator much bigger and more powerful than you, or struggling for your life inside his stomach. In this case, it could be many of those descriptions! Knowing Mordecai, these could be ordinary fat rats, or they could literally be big-business types that were turned into rats and are being devoured. He could probably tell us, but it's a little hard (and very rude) to talk with one's mouth full. So we'll have to discern what we can ourselves. That is, there's a lot of fat rats, and they're going to be turned into owl fat after a fat owl has swallowed them all down. At this point, you can see just how big his rat buffet is already making him, too! That's a lotta rats.

But hey, y'know. Run around, live your life, fight in pursuit of status, get turned into a rat (or be a rat), get eaten by an owl, become owl fat, that's just what life is, y'know?

Art by  ifus! Original over here: http://www.furaffinity.net/view/22692709/