Apart from a few pumpkins in the Main St windows and some Halloween themed merchandise in the stores, MK really doesn't have any decorations up during non-MNSSHP times. The decorations for the party nights are lots of lighting effects and some other easy to set up decorations that are only put out/blown up/turned on during the parties.
The only specail Halloween stuff I've ever seen at the resorts was Halloween themed displays in the shops. There might be a few more things the week of Halloween itself around and about, but there isn't a bunch of Halloween stuff all over like the Christmas decorations are during the holidays.