I have never heard of that. I did know Mardi Gras from a Paul Simon song, but I thought it was a pop festival.
As concerns the goings-on in New Orleans, yes.
However, the day before Ash Wednesday has long been an important event among the historic Christian churches, although not by that particular name (Fat Tuesday). That term only became common throughout the USA in recent years. Previously, and still as it is in the liturgical churches, the name for the day is Shrove Tuesday.
That name refers to people confessing their sins, stripping them away, and readying themselves for the start of the season of Lent in which they are supposed to share, symbolically, in Christ's 40 days of fasting in the wilderness through acts of self-denial.
Just the same, Shrove Tuesday has also been a day for eating hearty since none of that is supposed to go on during Lent.