Being from Louisiana I have visited Disney many times during Mardi Gras. I can say, without a doubt, that each year, regardless of when MG falls, it seems more and more crowded (like Disney in general these days).
It used to be the perfect time to go with low crowds and amazing weather. A few years back we visited for both MG and Thanksgiving week in the same year, and our MG trip was more crowded of the two! My sister in law and her family visited for MG this year, and after 10 or more trips to Disney she said this was the busiest she has ever seen it. She is a teacher, they only travel during holiday breaks and summer.
Having said all that, we will still continue to go at MG, mainly because of the weather. Just don't go thinking it's going to be low crowds simply because it's a holiday most of the country doesn't celebrate and that most schools aren't on break.