I think how crowded Disney is during Mardi Gras really depends on when Mardi Gras falls. Outside of Lousiana and some parts of Mississippi and Alabama, students typically do not have off for Mardi Gras and the holiday is more or less a non event. So Mardi Gras in itself typically will not have as big an impact on crowds as Presidents Day, Spring Break, etc.
This year with Mardi Gras falling the week before Presidents Day it will likely be less crowded early in the week but get crowded by Thursday/Friday when people start arriving for the Presidents Day weekend.