Are they all dressed up during the entire Christmas Season?

The ones in Camp Mickey/Minnie (DAK) were dressed and the background has a Christmas tree, also in AK in another location, Pooh and friends had on hats and scarves. They were not dressed in Crystal Palace. My memory is failing me for other locations right now.

They are dressed up for certain shows and such. If you go to the MVMCP at the MK you will definitely see the characters dressed up! Goofy is usually Santa, Pluto wears reindeer antlers. Mickey and Minnie are usually dressed in their holiday gear and you should see Donald Duck as well as Chip & Dale wearing their winter scarves and stocking caps! If you go to Epcot the characters are dressed up for the tree lighting ceremony which takes place nightly.

Have a wonderful time! There is nothing like being at WDW during the Christmas Holiday season!