I would love to dress up, but DH is pretty much against it. We have been to
MNSSHP 3 times and we have always worn long sleeved tshirts that have cute skeletons on them. I bought them at Old Navy. I imagine that will be our "costume" again this year. It has now become a tradition!
There really are not a lot of adults that dress up, but most kids do. I think you would have a blast in costume, but you also need to consider if you will feel uncomfortable if you are in the minority of those wearing or not wearing costumes.
One year I saw a man with a huge milk carton around his head with the "missing person" above his face...too funny!
Now...Halloween late night at PI is a different story...I would say that more than half the adults there are in costume. Sometimes they have contests and the costumes are simply astounding!
If you go to PI on that night (or any other) be sure to check out the Jazz Club if Toxic Audio is the headliner. They are great. They perform using only their voices to make musical instrument sounds. I hope they are performing this year.
With all this said...Halloween is my favorite time at Disney. I LOVE the boat ride from WL to MK and seeing the park all "spooky". It is a wonderful perspective from the boat. We won't do it this year since we are staying at BWV and I will definitely miss the ride over. It just sets the mood for the whole evening. We have even floated by the electrical parade on that ride!
Have Fun!