When I was looking after the info about the Fawful-like thing on Wikipedia (which I got), I found something interesting. Since I can't speak Japanese, I'm going to cite the wiki directly:
A mysterious plague is taking over the Mushroom Kingdom. The only symptom that this disease results in is the inflating of the Toads many times their normal size, causing them to grow big and roll around without any control; the disease is known as the "Metaknight"."
I didn't find anything on Wiktionary, so I'm pretty sure that this is a reference to the Kirby series character "Meta Knight" and his "Meta-Knights".
EDIT: Just saw that there already was a thread for M&LRPG3 cameos which I missed. Too bad this proxy sucks so much that it can't delete threades, so you gotta lock it.
It's a actually a cultural reference. Metabo is short for "metabolic syndrome," which is the latest concern/fad in Japan. Essentially, it refers to being fat and the associated health effects. Japan has even passed a law that says if your waistline is over a certain size, your employer or city is fined.
Koro koro is onomatopoeia for rolling (e.g. Kirby's Tilt and Tumble was called Koro Koro Kirby in Japan, and there's Kororinpa, the marble rolling game).
In M&L3, the Mushroom Kingdom citizens are afflicted with a disease that makes them swell into a ball and roll around; thus the disease, "Metakoro."