Yes. Even if we magic-handwave all of the crops dying and the world becoming too cold to survive on, NMM's rule would be grim:
1) Most ponies in the FiM universe seem similar to Earth ponies (the irl ones, not the show's Earth Ponies). This means they are diurnal creatures. If every single pony was forced to be nocturnal, their circadian rhythm would be screwy and while I'm sure some would adjust, many would probably end up tired and depressed all the time. I wonder if the bat winged/eared ponies seen in Luna and NMM's guard are nocturnal, perhaps by magic or a special bloodline?
2) We only got to glimpse at it for a couple of minutes but as others said, it appeared to be a dictatorship ruled with an iron fis-er, hoof. Even in that small window of time, the ponies appeared scared to even say Celestia and all of the armed guards looked like a military force. There are already enough real-world examples of such things to show how miserable living under those conditions would be.