Mardi Gras is fun, but it doesn't really add all that much. It was a nice extra when we went, but I wouldn't go out of my way just to be there during it. It isn't like an all day party or anything.
They have the parade at night and free concerts on the weekends. They also have an small area set up near the parade route where they sell "Mardi Gras" foods and souvenirs. You can also buy beads and Mardi Gras merchandise like shirts in the gift shops. That's about it.
As someone who has lived in New Orleans, the costumes and floats are every bit as good as the parades there are. They also throw plenty of beeds, and overall it didn't seem too crowded along the route, so it wasn't hard to see or get the beads.
They also have the stilt walkers out interacting with guests along the route before the parade starts, and that was very nice. We were able to get some very good pics, and the TMs performing were very funny and entertaining, playing with the kids, and playing jokes on them while we waited for the parade to start.
The only difference I really noticed is that at all the Mardi Gra parades in New Orleans I've been to the people on the floats throw all kinds of other little trinkets besides beads, and they also throw more of the fancy elaborate beads out. I've even gotten t-shirts before. Whereas at Universal, they just throw the standard plain beads, and you have to buy anything more elaborate at the gift shops.
The kids didn't seem to care though. They had so much fun collecting all the beads.