This is my first time completely covering a cake in fondant (MMF). I've used fondant before and I've watched the craftsy tutorial on how to cover a cake, but this is a total disaster. I frosted the cake with my usual buttercream and got it as smooth as possible. Sadly this batch of MMF has loads of bubbles for some reason but I decided to go ahead and use it to cover the cake because I will be attaching pieces that I figured would mostly hide it.  I rolled it out and got it on my cake and could tell right away it was mushing the frosting, even though I'd rolled it very thin. I went ahead and tried to smooth it, even though my corners are no longer sharp at all, and now there is a massive air bubble forming. What the heck do I do and why is this all happening?! 

I'd say the first problem is your frosting isn't hard enough. Did it have enough time to get hard/crust?

I did give it time to crust but for some reason this time it didn't crust as much as it usually does. Does this mean I need to add more powdered sugar? That it's not a stiff enough icing?

What makes icing crust is more sugar to fat ratio.  Usually that is created by using less fat to begin with.  Makes for a much tastier icing.  Try the "2 of everything icing" recipe that can be found on this site.

Pop the air bubble with a sharp (and clean) pin.  hole in the middle of it and smooth inwards towards the hole to remove air.  Smooth over after.  I've never made MMF so can't help on why it's happening but popping should fix it.