ER, the area? Mardi Gras is the day before Ash Wednesday. It isn't a place. It's a splurge before the start of Lent.
Among the biggest celebrations worldwide: Rio; Cologne, Germany; Venice; Quebec City, Canada; and even Goa, India! Multiple cultures around the world celebrate more simply with a special donut, pancake or split pea soup. You can get donuts pretty much everywhere.
Where WDW is concerned, threads like this one pop up every year on Mardi Gras week.
I suspect crowd prediction sites overlook it ever year because the date varies year to year. The prediction sites use a flawed formula, which probably also explains why their Easter numbers are often low.
Crowds are also relative to cost. Many families are happy to make a trade off between cost/crowds, and how much school their kids will miss.
A February trip can easily cost $1,000's less than an Easter week trip. Folks on the DIS love to talk about search up low-crowd weeks, but that isn't good for WDW, the airlines, car rental companies, etc. All of these businesses prefer even crowds. Nowadays, all of the above are using better and better pricing algorithms to better accomplish that goal.