As for the prerequisite of skills, you need to add Ranks to the requirements. Since you can't take the Awesome Costume Feat until 6th level (it's a requirement of the feat), you can't enter this class until that level.

I'd recommend requiring these skills: Bluff, Disguise, Profession (Tailor or Seamstress) and making these skills class skills for the Pumpkin King. Additional class skills would be: Diplomacy, Intimidate, Knowledge (Local), and Move Silently.

Custom Costume could use a little work. For example, you neglect to say how far you get Darkvision. It should be 30 feet. If you already have Darkvision, you should gain an additional 30 feet of Darkvision.

Natural Armor +1 probably shouldn't be based on the number of the Halloween feats you have. You need a minimum of three Halloween feats just to enter the class. Alternatively, you could gain a natural armor bonus equal to half your level Pumpkin King (rounded down).

Templates normally can only be added to creatures. Allowing them to be added to a costume sounds odd to me. Plus, this risks the costume being significantly overpowered. If nothing else, it should be at the DM's discretion.

Debby