I made MMF for the first time on Friday night and let it sit overnight covered with a thin layer of Crisco, double wrapped in plastic wrap and stored in a ziploc bag. It felt nice and elastic, rolled out nicely with some cornstarch on the counter and rolling pin, lifted nicely onto the cake, draped nicely and stuck to the buttercream well.
The problems are that #1 there were cornstarch spots on my blue fondant that I couldn't brush off and #2 that my fondant wasn't as matte as I would have liked. When I first put it on the cake it wasn't shiny but as the day went by it lost most of it's matte appearance and looked more like an eggshell painted wall - semi shiny but definitely not catching a ton of light.
How do I avoid those issues in the future?
Did you put it on a cold cake? Sometimes that will make your fondant turn shiny.
For the cornstarch spots, brush it with a clean pastry brush. If that doesn't do the trick, then paint it with vodka...but that will definitely make it shiny.
If you have and airbrush, you can spray a thin coat of vodka to eliminate the white dusty look. Not sure about the shine factor though.